Electrons are tiny, negatively charged particles that form a cloud around the nucleus of an atom. Each electron carries a single fundamental unit of negative electric charge, or -1.

The electron is one of the lightest particles with a known mass. A droplet of water weighs about a billion, billion, billion times more than an electron. Physicists believe that electrons are one of the fundamental particles of physics, which means they cannot be split into anything smaller. Physicists also believe that electrons do not have any real size, but are instead true points in space-that is, an electron has a radius of zero.

Electrons act differently than everyday objects because electrons can behave as both particles and waves. Actually, all objects have this property, but the wavelike behavior of larger objects, such as sand, marbles, or even people, is too small to measure. In very small particles wave behavior is measurable and important. Electrons travel around the nucleus of an atom, but because they behave like waves, they do not follow a specific path like a planet orbiting the Sun does. Instead they form regions of negative electric charge around the nucleus. These regions are called orbitals, and they correspond to the space in which the electron is most likely to be found. As we will discuss later, orbitals have different sizes and shapes, depending on the energy of the electrons occupying them.

Protons carry a positive charge of +1, exactly the opposite electric charge as electrons. The number of protons in the nucleus determines the total quantity of positive charge in the atom. In an electrically neutral atom, the number of the protons and the number of electrons are equal, so that the positive and negative charges balance out to zero. The proton is very small, but it is fairly massive compared to the other particles that make up matter. A proton’s mass is about 1,840 times the mass of an electron.

Neutrons are about the same size as protons but their mass is slightly greater. Without neutrons present, the repulsion among the positively charged protons would cause the nucleus to fly apart. Consider the element helium, which has two protons in its nucleus. If the nucleus did not contain neutrons as well, it would be unstable because of the electrical repulsion between the protons. (The process by which neutrons hold the nucleus together is explained below in the Strong Force section of this article.) A helium nucleus needs either one or two neutrons to be stable. Most atoms are stable and exist for a long period of time, but some atoms are unstable and spontaneously break apart and change, or decay, into other atoms.

Unlike electrons, which are fundamental particles, protons and neutrons are made up of other, smaller particles called quarks. Physicists know of six different quarks. Neutrons and protons are made up of up quarks and down quarks-two of the six different kinds of quarks. The fanciful names of quarks have nothing to do with their properties; the names are simply labels to distinguish one quark from another.

Quarks are unique among all elementary particles in that they have electric charges that are fractions of the fundamental charge. All other particles have electric charges of zero or of whole multiples of the fundamental charge. Up quarks have electric charges of +’. Down quarks have charges of -€. A proton is made up of two up quarks and a down quark, so its electric charge is ’ + ’ - €, for a total charge of +1. A neutron is made up of an up quark and two down quarks, so its electric charge is ’ - € - €, for a net charge of zero. Physicists believe that quarks are true fundamental particles, so they have no internal structure and cannot be split into something smaller.

Atoms have several properties that help distinguish one type of atom from another and determine how atoms change under certain conditions.

Each element has a unique number of protons in its atoms. This number is called the atomic number (abbreviated Z). Because atoms are normally electrically neutral, the atomic number also specifies how many electrons an atom will have. The number of electrons, in turn, determines many of the chemical and physical properties of the atom. The lightest atom, hydrogen, has an atomic number equal to one, contains one proton, and (if electrically neutral) one electron. The most massive stable atom found in nature is bismuth (Z = 83). More massive unstable atoms also exist in nature, but they break apart and change into other atoms over time. Scientists have produced even more massive unstable elements in laboratories.

The total number of protons and neutrons in the nucleus of an atom is the mass number of the atom (abbreviated A). The mass number of an atom is an approximation of the mass of the atom. The electrons contribute very little mass to the atom, so they are not included in the mass number. A stable helium atom can have a mass number equal to three (two protons plus one neutron) or equal to four (two protons plus two neutrons). Bismuth, with 83 protons, requires 126 neutrons for stability, so its mass number is 209 (83 protons plus 126 neutrons).

Scientists usually measure the mass of an atom in terms of a unit called the atomic mass unit (abbreviated amu). They define an amu as exactly 1/12 the mass of an atom of carbon with six protons and six neutrons. On this scale, the mass of a proton is 1.00728 amu and the mass of a neutron is 1.00866 amu. The mass of an atom measured in amu is nearly equal to its mass number.

Scientists can use a device called a mass spectrometer to measure atomic mass. A mass spectrometer removes one or more electrons from an atom. The electrons are so light that removing them hardly changes the mass of the atom at all. The spectrometer then sends the atom through a magnetic field, a region of space that exerts a force on magnetic or electrically charged particles. Because of the missing electrons, the atom has more protons than electrons and hence a net positive charge. The magnetic field bends the path of the positively charged atom as it moves through the field. The amount of bending depends on the atom’s mass. Lighter atoms will be affected more strongly than heavier atoms. By measuring how much the atom’s path curves, a scientist can determine the atom’s mass.

The atomic mass of an atom, which depends on the number of protons and neutrons present, also relates to the atomic weight of an element. Weight usually refers to the force of gravity on an object, but atomic weight is really just another way to express mass. An element’s atomic weight is given in grams. It represents the mass of one mole (6.02 × 1023 atoms) of that element. Numerically, the atomic mass and the atomic weight of an element are the same, but the first is expressed in grams and the second is in atomic mass units. So, the atomic weight of hydrogen is 1 gram and the atomic mass of hydrogen is 1 amu.

Atoms of the same element that differ in mass number are called isotopes. Since all atoms of a given element have the same number of protons in their nucleus, isotopes must have different numbers of neutrons. Helium, for example, has an atomic number of 2 because of the two protons in its nucleus. But helium has two stable isotopes-one with one neutron in the nucleus and a mass number equal to three and another with two neutrons and a mass number equal to four.

Scientists attach the mass number to an element’s name to differentiate between isotopes. Under this convention, helium with a mass number of three is called helium-3, and helium with a mass number of four is called helium-4. Helium in its natural form on Earth is a mixture of these two isotopes. The percentage of each isotope found in nature is called the isotope’s isotopic abundance. The isotopic abundance of helium-3 is very small, only 0.00014 percent, while the abundance of helium-4 is 99.99986 percent. This means that only about one of every 1 million helium atoms is helium-3, and the rest are all helium-4. Bismuth has only one naturally occurring stable isotope, bismuth-209. Bismuth-209’s isotopic abundance is therefore 100 percent. The element with the largest number of stable isotopes found in nature is tin, which has ten stable isotopes.

All elements also have unstable isotopes, which are more susceptible to breaking down, or decaying, than are the other isotopes of an element. When atoms decay, the number of protons in their nucleus changes. Since the number of protons in the nucleus of an atom determines what element that atom belongs to, this decay changes one element into another. Different isotopes decay at different rates. One way to measure the decay rate of an isotope is to find its half-life. An isotope’s half-life is the time that passes until half of a sample of an isotope has decayed.

The various isotopes of a given element have nearly identical chemical properties and many similar physical properties. They differ, of course, in their mass. The mass of a helium-3 atom, for example, is 3.016 amu, while the mass of a helium-4 atom is 4.003 amu.

Usually scientists do not specify the atomic weight of an element in terms of one isotope or another. Instead, they express atomic weight as an average of all of the naturally occurring isotopes of the element, taking into account the isotopic abundance of each. For example, the element copper has two naturally occurring isotopes: copper-63, with a mass of 62.930 amu and an isotopic abundance of 69.2 percent, and copper-65, with a mass of 64.928 amu and an abundance of 30.8 percent. The average mass of naturally occurring copper atoms is equal to the sum of the atomic mass for each isotope multiplied by its isotopic abundance. For copper, it would be (62.930 amu x 0.692) + (64.928 amu x 0.308) = 63.545 amu. The atomic weight of copper is therefore 63.545 g.

About 300 combinations of protons and neutrons in nuclei are stable enough to exist in nature. Scientists can produce another 3,000 nuclei in the laboratory. These nuclei tend to be extremely unstable because they have too many protons or neutrons to stay in one piece for long. Unstable nuclei, whether naturally occurring or created in the laboratory, break apart or change into stable nuclei through a variety of processes known as radioactive decays (see Radioactivity).

Some nuclei with an excess of protons simply eject a proton. A similar process can occur in nuclei with an excess of neutrons. A more common process of decay is for a nucleus to simultaneously eject a cluster of 2 protons and 2 neutrons. This cluster is actually the nucleus of an atom of helium-4, and this decay process is called alpha decay. Before scientists identified the ejected particle as a helium-4 nucleus, they called it an alpha particle. Helium-4 nuclei are still sometimes called alpha particles.

The most common way for a nucleus to get rid of excess protons or neutrons is to convert a proton into a neutron or a neutron into a proton. This process is known as beta decay. The total electric charge before and after the decay must remain the same. Because protons are electrically charged and neutrons are not, the reaction must involve other charged particles. For example, a neutron can decay into a proton, an electron, and another particle called an electron antineutrino. The neutron has no charge, so the charge at the beginning of the reaction is zero. The proton has an electric charge of +1 and the electron has an electric charge of -1. The antineutrino is a tiny particle with no electric charge. The electric charges of the proton and electron cancel each other, leaving a net charge of zero. The electron is the most easily detected product of this type of beta decay, and scientists called these products beta particles before they identified them as electrons.

Beta decay also results when a proton changes to a neutron. The end result of this decay must have a charge of +1 to balance the charge of the initial proton. The proton changes into a neutron, an anti-electron (also called a positron), and an electron neutrino. A positron is identical to an electron, except the positron has an electric charge of +1. The electron neutrino is a tiny, electrically neutral particle. The difference between the antineutrino in neutron-proton beta decay and the neutrino in proton-neutron beta decay is very subtle-so subtle that scientists have yet to prove that a difference actually exists.

While scientists often create unstable nuclei in the laboratory, several radioactive isotopes also occur naturally. These atoms decay more slowly than most of the radioactive isotopes created in laboratories. If they decayed too rapidly, they wouldn’t stay around long enough for scientists to find them. The heavy radioactive isotopes found on Earth formed in the interiors of stars more than 5 billion years ago. They were part of the cloud of gas and dust that formed our solar system and, as such, are reminders of the origin of Earth and the other planets. In addition, the decay of radioactive material provides much of the energy that heats Earth’s core.

The most common naturally occurring radioactive isotopes are potassium-40 (see Potassium), thorium-232 (see Thorium), and uranium-238 (see Uranium). Atoms of these isotopes last, on average, for billions of years before undergoing alpha or beta decay. The steady decay of these isotopes and other, more stable atoms allows scientists to determine the age of minerals in which these isotopes occur. Scientists begin by estimating the amount of isotope that was present when the mineral formed, then measure how much has decayed. Knowing the rate at which the isotope decays, they can determine how much time has passed. This process, known as radioactive dating (see Dating Methods), allows scientists to measure the age of Earth. The currently accepted value for Earth’s age is about 4.5 billion years. Scientists have also examined rocks from the Moon and other objects in the solar system and have found that they have similar ages.

In physics, a force is a push or pull on an object. There are four fundamental forces, three of which-the electromagnetic force, the strong force, and the weak force-are involved in keeping stable atoms in one piece and determining how unstable atoms will decay. The electromagnetic force keeps electrons attached to their atom. The strong force holds the protons and neutrons together in the nucleus. The weak force governs how atoms decay when they have excess protons or neutrons. The fourth fundamental force, gravity, only becomes apparent with objects much larger than subatomic particles.

The most familiar of the forces at work inside the atom is the electromagnetic force. This is the same force that causes people’s hair to stick to a brush or comb when they have a buildup of static electricity. The electromagnetic force causes opposite electric charges to attract each other. Because of this force, the negatively charged electrons in an atom are attracted to the positively charged protons in the atom’s nucleus. This force of attraction binds the electrons to the atom. The electromagnetic force becomes stronger as the distance between charges becomes smaller. This property usually causes oppositely charged particles to come as close to each other as possible. For many years, scientists wondered why electrons didn’t just spiral into the nucleus of an atom, getting as close as possible to the protons. Physicists eventually learned that particles as small as electrons can behave like waves, and this property keeps electrons at set distances from the atom’s nucleus. The wavelike nature of electrons is discussed below in the Quantum Atom section of this article.

The electromagnetic force also causes like charges to repel each other. The negatively charged electrons repel one another and tend to move far apart from each other, but the positively charged nucleus exerts enough electromagnetic force to keep the electrons attached to the atom. Protons in the nucleus also repel one other, but, as described below, the strong force overcomes the electromagnetic force in the nucleus to hold the protons together.

Protons and neutrons in the nuclei of atoms are held together by the strong force. This force must overcome the electromagnetic force of repulsion the protons in a nucleus exert on one another. The strong force that occurs between protons alone, however, is not enough to hold them together. Other particles that add to the strong force, but not to the electromagnetic force, must be present to make a nucleus stable. The particles that provide this additional force are neutrons. Neutrons add to the strong force of attraction but have no electric charge and so do not increase the electromagnetic repulsion.

The strong force only operates at very short range-about 2 femtometers (abbreviated fm), or 2 × 10-15 m (8 × 10-14 in). Physicists also use the word fermi (also abbreviated fm) for this unit in honor of Italian-born American physicist Enrico Fermi. The short-range property of the strong force makes it very different from the electromagnetic and gravitational forces. These latter forces become weaker as distance increases, but they continue to affect objects millions of light-years away from each other. Conversely, the strong force has such limited range that not even all protons and neutrons in the same nucleus feel each other’s strong force. Because the diameter of even a small nucleus is about 5 to 6 fm, protons and neutrons on opposite sides of a nucleus only feel the strong force from their nearest neighbors.

The strong force differs from electromagnetic and gravitational forces in another important way-the way it changes with distance. Electromagnetic and gravitational forces of attraction increase as particles move closer to one another, no matter how close the particles get. This increase causes particles to move as close together as possible. The strong force, on the other hand, remains roughly constant as protons and neutrons move closer together than about 2 fm. If the particles are forced much closer together, the attractive nuclear force suddenly turns repulsive. This property causes nuclei to form with the same average spacing-about 2 fm-between the protons and neutrons, no matter how many protons and neutrons there are in the nucleus.

The unique nature of the strong force determines the relative number of protons and neutrons in the nucleus. If a nucleus has too many protons, the strong force cannot overcome the electromagnetic repulsion of the protons. If the nucleus has too many neutrons, the excess strong force tries to crowd the protons and neutrons too close together. Most stable atomic nuclei fall between these extremes. Lighter nuclei, such as carbon-12 and oxygen-16, are made up of 50 percent protons and 50 percent neutrons. More massive nuclei, such as bismuth-209, contain about 40 percent protons and 60 percent neutrons.

Particle physicists explain the behavior of the strong force by introducing another type of particle, called a pion. Protons and neutrons interact in the nucleus by exchanging pions. Exchanging pions pulls protons and neutrons together. The process is similar to two people having a game of catch with a heavy ball, but with each person attached to the ball by a spring. As one person throws the ball to the other, the spring pulls the thrower toward the ball. If the players exchange the ball rapidly enough, the ball and springs become just a blur to an observer, and it appears as if the two throwers are simply pulled toward one another. This is what occurs in the nuclei of atoms. The protons and neutrons in the nucleus are the people, pions act as the ball, and the strong force acts as the springs holding everything together.

Pions in the nucleus exist only for the briefest instant of time, no more than 1 × 10-23 seconds, but even during their short existence they can provide the attraction that holds the nucleus together. Pions can also exist as independent particles outside of the nucleus of an atom. Scientists have created them by striking high-speed protons against a target. Even though the free pions also live only for a short period of time (about 1 × 10-8 seconds), scientists have been able study their properties.

The weak force lives up to its name-it is much weaker than the electromagnetic and strong forces. Like the strong force, it only acts over a short distance, about .01 fm. Unlike these other forces, however, the weak force affects all the particles in an atom. The electromagnetic force only affects the electrons and protons, and the strong force only affects the protons and neutrons. When a nucleus has too many protons to hold together or so many neutrons that the strong force squeezes too tightly, the weak force actually changes one type of particle into another. When an atom undergoes one type of decay, for example, the weak force causes a neutron to change into a proton, an electron, and an electron antineutrino. The total electric charge and the total energy of the particles remain the same before and after the change.

Scientists of the early 20th century found they could not explain the behavior of atoms using their current knowledge of matter. They had to develop a new view of matter and energy to accurately describe how atoms behaved. They called this theory quantum theory, or quantum mechanics. Quantum theory describes matter as acting both as a particle and as a wave. In the visible objects encountered in everyday life, the wavelike nature of matter is too small to be apparent. Wavelike nature becomes important, however, in microscopic particles such as electrons. As we have discussed, electrons in atoms behave like waves. They exist as a fuzzy cloud of negative charge around the nucleus, instead of as a particle located at a single point.

In order to understand the quantum model of the atom, we must know some basic facts about waves. Waves are vibrations that repeat regularly over and over again. A familiar example of waves occurs when one end of a rope is tied to a fixed object and someone moves the other end up and down. This action creates waves that travel along the rope. The highest point that the rope reaches is called the crest of the wave. The lowest point is called the trough of the wave. Troughs and crests follow each other in a regular sequence. The distance from one trough to the next trough, or from one crest to the next crest, is called a wavelength. The number of wavelengths that pass a certain point in a given amount of time is called the wave’s frequency.

In physics, the word wave usually means the entire pattern, which may consist of many individual troughs and crests. For example, when the person holding the loose end of the rope moves it up and down very fast, many troughs and crests occupy the rope at once. A physicist would use the word wave to describe the entire set of troughs and crests on the rope.

When two waves meet each other, they merge in a process called interference. Interference creates a new wave pattern. If two waves with the same wavelength and frequency come together, the resulting pattern depends on the relative position of the waves’ crests. If the crests and troughs of the two waves coincide, the waves are said to be in phase. Waves in phase with each other will merge to produce higher crests and lower troughs. Physicists call this type of interference constructive interference.

Sometimes waves with the same wavelength and frequency are out of phase, meaning they meet in such a way that their respective crests and troughs do not coincide. In these cases the waves produce destructive interference. If two identical waves are exactly half a wavelength out of phase, the crests of one wave line up with the troughs of the other. These waves cancel each other out completely, and no wave will appear. If two waves meet that are not exactly in phase and not exactly one-half wavelength out of phase, they will interfere constructively in some places and destructively in others, producing a complicated new wave. See also Wave Motion.

Electrons behave as both particles and waves in atoms. This characteristic is called wave-particle duality. Wave-particle duality actually affects all particles and collections of particles, including protons, neutrons, and atoms themselves. But in terms of the structure of the atom, the wavelike nature of the electron is the most important.

As waves, electrons have wavelengths and frequencies. The wavelength of an electron depends on the electron’s energy. Since the energy of electrons is kinetic (energy related to motion), an electron’s wavelength depends on how fast it is moving. The more energy an electron has, the shorter its wavelength is. Electron waves can interfere with each other, just as waves along a rope do.

Because of the electron’s wave-particle duality, physicists cannot define an electron’s exact location in an atom. If the electron were just a particle, measuring its location would be relatively simple. As soon as physicists try to measure its location, however, the electron’s wavelike nature becomes apparent, and they cannot pinpoint an exact location. Instead, physicists calculate the probability that the electron is located in a certain place. Adding up all these probabilities, physicists can produce a picture of the electron that resembles a fuzzy cloud around the nucleus. The densest part of this cloud represents the place where the electron is most likely to be located.

Physicists call the region of space an electron occupies in an atom the electron’s orbital. Similar orbitals constitute groups called shells. The electrons in the orbitals of a particular shell have similar levels of energy. This energy is in the form of both kinetic energy and potential energy. Lower shells are close to the nucleus and higher shells are farther from the nucleus. Electrons occupying orbitals in higher shells generally have more energy than electrons occupying orbitals in lower shells.

The wavelike nature of electrons sets boundaries for their possible locations and determines what shape their orbital, or cloud of probability, will form. Orbitals differ from each other in size, angular momentum, and magnetic properties. In general, angular momentum is the energy an object contains based on how fast the object is revolving, the object’s mass, and the object’s distance from the axis around which it is revolving. The angular momentum of a whirling ball tied to a string, for example, would be greater if the ball was heavier, the string was longer, or the whirling was faster. In atoms, the angular momentum of an electron orbital depends on the size and shape of the orbital. Orbitals with the same size and shape all have the same angular momentum. Some orbitals, however, can differ in shape but still have the same angular momentum. The magnetic properties of an orbital describe how it would behave in a magnetic field. Magnetic properties also depend on the size and shape of the orbital, as well as on the orbital’s orientation in space.

The orbitals in an atom must occur at certain distances from the nucleus to create a stable atom. At these distances, the orbitals allow the electron wave to complete one or more half-wavelengths (y, 1, 1y, 2, 2y, and so on) as it travels around the nucleus. The electron wave can then double back on itself and constructively interfere with itself in a way that reinforces the wave. Any other distance would cause the electron to interfere with its own wave in an unpredictable and unstable way, creating an unstable atom.

Physicists call the number of half-wavelengths that an orbital allows the orbital’s principal quantum number (abbreviated n). In general, this number determines the size of the orbital. Larger orbitals allow more half-wavelengths and therefore have higher principal quantum numbers. The orbital that allows one half-wavelength has a principal quantum number of one. Only one orbital allows one half-wavelength. More than one orbital can allow two or more half-wavelengths. These orbitals may have the same principal quantum number, but they differ from each other in their angular momentum and their magnetic properties. The orbitals that allow one wavelength have a principal quantum number of 2 (n = 2), the orbitals that allow one and a half wavelengths have a principal quantum number of 3 (n = 3), and so on. The set of orbitals with the same principal quantum number make up a shell.

Physicists use a second number to describe the angular momentum of an orbital. This number is called the orbital’s secondary quantum number, or its angular momentum quantum number (abbreviated l). The number of possible values an orbital can have for its angular momentum is one less than the number of half-wavelengths it allows. This means that an orbital with a principal quantum number of n can have n-1 possible values for its secondary quantum number.

Physicists customarily use letters to indicate orbitals with certain secondary quantum numbers. In order of increasing angular momentum, the orbitals with the six lowest secondary quantum numbers are indicated by the letters s, p, d, f, g, and h. The letter s corresponds to the secondary quantum number 0, the letter p corresponds to the secondary quantum number 1, and so on. In general, the angular momentum of an orbital depends on its shape. An s-orbital, with a secondary quantum number of 0, is spherical. A p-orbital, with a secondary quantum number of 1, resembles two hemispheres, facing one another. The possible combinations of principal and secondary quantum numbers for the first five shells are listed below.

More than one orbital can allow the same number of half-wavelengths and have the same angular momentum. Physicists call orbitals in a shell that all have the same angular momentum a subshell. They designate a subshell with the subshell’s principal and secondary quantum numbers. For example, the 1s subshell is the group of orbitals in the first shell with an angular momentum described by the letter s. The 2p subshell is the group of orbitals in the second shell with an angular momentum described by the letter p.

Orbitals within a subshell differ from each other in their magnetic properties. The magnetic properties of an orbital depend on its shape and orientation in space. For example, a p-orbital can have three different orientations in space: one situated up and down, one from side to side, and a third from front to back.

Physicists describe the magnetic properties of an orbital with a third quantum number called the orbital’s magnetic quantum number (abbreviated m). The magnetic quantum number determines how orbitals with the same size and angular momentum are oriented in space. An orbital’s magnetic quantum number can only have whole number values ranging from the value of the orbital’s secondary quantum number down to the negative value of the secondary quantum number. A p-orbital, for example, has a secondary quantum number of 1 (l = 1), so the magnetic quantum number has three possible values: +1, 0, and -1. This means the p-orbital has three possible orientations in space. An s-orbital has a secondary quantum number of 0 (l = 0), so the magnetic quantum number has only one possibility: 0. This orbital is a sphere, and a sphere can only have one orientation in space. For a d-orbital, the secondary quantum number is 2 (l = 2), so the magnetic quantum number has five possible values: -2, -1, 0, +1, and +2. A d-orbital has four possible orientations in space, as well as a fifth orbital that differs in shape from the other four. Together, the principal, secondary, and magnetic quantum numbers specify a particular orbital in an atom.

Electrons are a type of particle known as a fermion. Austrian-American physicist Wolfgang Pauli discovered that no two fermions can have the exact same quantum numbers. This principle is called the Pauli exclusion principle, which states that two or more identical electrons cannot occupy the same orbital in an atom. Scientists know, however, that each orbital can hold two electrons. Electrons have another property, called spin, that differentiates the two electrons in each orbital. An electron’s spin has two possible values: +y (called spin-up) or -y (called spin-down). These two possible values mean that two electrons can occupy the same orbital, as long as their spins are different. Physicists call spin the fourth quantum number of an electron orbital (abbreviated ms). Spin, in addition to the other three quantum numbers, uniquely describes a particular electron’s orbital.

When electrons collect around an atom’s nucleus, they fill up orbitals in a definite pattern. They seek the first available orbital that takes the least amount of energy to occupy. Generally, it takes more energy to occupy orbitals with higher quantum numbers. It takes the same energy to occupy all the orbitals in a subshell. The lowest energy orbital is the one closest to the nucleus. It has a principal quantum number of 1, a secondary quantum number of 0, and a magnetic quantum number of 0. The first two electrons-with opposite spins-occupy this orbital.

If an atom has more than two electrons, the electrons begin filling orbitals in the next subshell with one electron each until all the orbitals in the subshell have one electron. The electrons that are left then go back and fill each orbital in the subshell with a second electron with opposite spin. They follow this order because it takes less energy to add an electron to an empty orbital than to complete a pair of electrons in an orbital. The electrons fill all the subshells in a shell, then go on to the next shell. As the subshells and shells increase, the order of energy for orbitals becomes more complicated. For example, it takes slightly less energy to occupy the s-subshell in the fourth shell than it does to occupy the d-subshell in the third shell. Electrons will therefore fill the orbitals in the 4s subshell before they fill the orbitals in the 3d subshell, even though the 3d subshell is in a lower shell.

The atom’s electron cloud, that is, the arrangement of electrons around an atom, determines most of the atom’s physical and chemical properties. Scientists can therefore predict how atoms will interact with other atoms by studying their electron clouds. The electrons in the outermost shell largely determine the chemical properties of an atom. If this shell is full, meaning all the orbitals in the shell have two electrons, then the atom is stable, and it won’t react readily with other atoms. If the shell is not full, the atom will chemically react with other atoms, exchanging or sharing electrons in order to fill its outer shell. Atoms bond with other atoms to fill their outer shells because it requires less energy to exist in this bonded state. Atoms always seek to exist in the lowest energy state possible.

Physicists call the outer shell of an atom its valence shell. The valence shell determines the atom’s chemical behavior, or how it reacts with other elements. The fullness of an atom’s valence shell affects how the atom reacts with other atoms. Atoms with valence shells that are completely full are not likely to interact with other atoms. Six gaseous elements-helium, neon, argon, krypton, xenon, and radon-have full valence shells. These six elements are often called the noble gases because they do not normally form compounds with other elements. The noble gases are chemically inert because their atoms are in a state of low energy. A full valence shell, like that of atoms of noble gases, provides the lowest and most stable energy for an atom.

Atoms that do not have a full valence shell try to lower their energy by filling up their valence shell. They can do this in several ways: Two atoms can share electrons to complete the valence shell of both atoms, an atom can shed or take on electrons to create a full valence shell, or a large number of atoms can share a common pool of electrons to complete their valence shells.

When two atoms share a pair of electrons, they form a covalent bond. When atoms bond covalently, they form molecules. A molecule can be made up of two or more atoms, all joined with covalent bonds. Each atom can share its electrons with one or more other atoms. Some molecules contain chains of thousands of covalently bonded atoms.

Carbon is an important example of an element that readily forms covalent bonds. Carbon has a total of six electrons. Two of the electrons fill up the first orbital, the 1s orbital, which is the only orbital in the first shell. The rest of the electrons partially fill carbon’s valence shell. Two fill up the next orbital, the 2s orbital, which forms the 2s subshell. Carbon’s valence shell still has the 2p subshell, containing three p-orbitals. The two remaining electrons each fill half of the two orbitals in the 2p subshell. The carbon atom thus has two half-full orbitals and one empty orbital in its valence shell. A carbon atom fills its valence shell by sharing electrons with other atoms, creating covalent bonds. The carbon atom can bond with other atoms through any of the three unfilled orbitals in its valence shell. The three available orbitals in carbon’s valence shell enable carbon to bond with other atoms in many different ways. This flexibility allows carbon to form a great variety of molecules, which can have a similarly great variety of geometrical shapes. This diversity of carbon-based molecules is responsible for the importance of carbon in molecules that form the basis for living things (see Organic Chemistry).

Atoms can also lose or gain electrons to complete their valence shell. An atom will tend to lose electrons if it has just a few electrons in its valence shell. After losing the electrons, the next lower shell, which is full, becomes its valence shell. An atom will tend to steal electrons away from other atoms if it only needs a few more electrons to complete the shell. Losing or gaining electrons gives an atom a net electric charge because the number of electrons in the atom is no longer the same as the number of protons. Atoms with net electric charge are called ions. Scientists call atoms with a net positive electric charge cations (pronounced CAT-eye-uhns) and atoms with a net negative electric charge anions (pronounced AN-eye-uhns).

The oppositely charged cations and anions are attracted to each other by electromagnetic force and form ionic bonds. When these ions come together, they form crystals. A crystal is a solid material made up of repeating patterns of atoms. Alternating positive and negative ions build up into a solid lattice, or framework. Crystals are also called ionic compounds, or salts.

The element sodium is an example of an atom that has a single electron in its valence shell. It will easily lose this electron and become a cation. Chlorine atoms are just one electron away from completing their valence shell. They will tend to steal an electron away from another atom, forming an anion. When sodium and chlorine atoms come together, the sodium atoms readily give up their outer electron to the chlorine atoms. The oppositely charged ions bond with each other to form the crystal known as sodium chloride, or table salt. See also Chemical Reaction.

Atoms can complete their valence shells in a third way: by bonding together in such a way so that all the atoms in the substance share each other’s outer electrons. This is the way metallic elements bond and fill their valence shells. Metals form crystal lattice structures similar to salts, but the outer electrons in their atoms do not belong to any atom in particular. Instead, the outer electrons belong to all the atoms in the crystal, and they are free to move throughout the crystal. This property makes metals good conductors of electricity.

The organization of the periodic table reflects the way elements fill their orbitals with electrons. Scientists first developed this chart by grouping together elements that behave similarly in order of increasing atomic number. Scientists eventually realized that the chemical and physical behavior of elements was dependant on the electron clouds of the atoms of each element. The periodic table does not have a simple rectangular shape. Each column lists elements that share chemical properties, properties that depend on the arrangement of electrons in the orbitals of atoms. These elements have the same number of electrons in their valence shells. Different numbers of elements have similar valence shells, so the columns of the periodic table differ in height. The noble gases are all located in the rightmost column of the periodic table, labeled column 18 in Encarta’s periodic table. The noble gases all have full valence shells and are extremely stable. The column labeled 11 holds the elements copper, silver, and gold. These elements are metals that have partially filled valence shells and conduct electricity well.

Each electron in an atom has a particular energy. This energy depends on the electron’s speed, the presence of other electrons, the electron’s distance from the nucleus, and the positive charge of the nucleus. For atoms with more than one electron, calculating the energy of each electron becomes too complicated to be practical. However, the order and relative energies of electrons follows the order of the electron orbitals, as discussed in the Electron Orbital and Shell section of this article. Physicists call the energy an electron has in a particular orbital the energy state of the electron. For example, the 1s orbital holds the two electrons with the lowest possible energies in an atom. These electrons are in the lowest energy state of any electrons in the atom.

When an atom gains or loses energy, it does so by adding energy to, or removing energy from, its electrons. This change in energy causes the electrons to move from one orbital, or allowed energy state, to another. Under ordinary conditions, all electrons in an atom are in their lowest possible energy states, given that only two electrons can occupy each orbital. Atoms gain energy by absorbing it from light or from a collision with another particle, or they gain it by entering an electric or magnetic field. When an atom absorbs energy, one or more of its electrons moves to a higher, or more energetic, orbital. Usually atoms can only hold energy for a very short amount of time-typically 1 × 10-12 seconds or less. When electrons drop back down to their original energy states, they release their extra energy in the form of a photon (a packet of radiation). Sometimes this radiation is in the form of visible light. The light emitted by a fluorescent lamp is an example of this process.

The outer electrons in an atom are easier to move to higher orbitals than the electrons in lower orbitals. The inner electrons require more energy to move because they are closer to the nucleus and therefore experience a stronger electromagnetic pull toward the nucleus than the outer electrons. When an inner electron absorbs energy and then falls back down, the photon it emits has more energy than the photon an outer electron would emit. The emitted energy relates directly to the wavelength of the photon. Photons with more energy are made of radiation with a shorter wavelength. When inner electrons drop down, they emit high-energy radiation, in the range of an X ray. X rays have much shorter wavelengths than visible light. When outer electrons drop down, they emit light with longer wavelengths, in the range of visible light.

Physicists and chemists first learned about the properties of atoms indirectly, by studying the way that atoms join together in molecules or how atoms and molecules make up solids, liquids, and gases. Modern devices such as electron microscopes, particle traps, spectroscopes, and particle accelerators allow scientists to perform experiments on small groups of atoms and even on individual atoms. Scientists use these experiments to study the properties of atoms more directly.

One of the most direct ways to study an object is to take its photograph. Scientists take photographs of atoms by using an electron microscope. An electron microscope imitates a normal camera, but it uses electrons instead of visible light to form an image. In photography, light reflects off of an object and is recorded on film or some other kind of detector. Taking a photograph of an atom with light is difficult because atoms are so tiny. Light, like all waves, tends to diffract, or bend around objects in its path (see Diffraction). In order to take a sharp photograph of any object, the wavelength of the light that bounces off the object must be much smaller than the size of the object. If the object is about the same size as or smaller than the light’s wavelength, the light will bend around the object and produce a fuzzy image.

Atoms are so small that even the shortest wavelengths of visible light will diffract around them. Therefore, capturing photographic images of atoms requires the use of waves that are shorter than those of visible light. X rays are a type of electromagnetic radiation like visible light, but they have very short wavelengths-much too short to be visible to human eyes. X-ray wavelengths are small enough to prevent the waves from diffracting around atoms. X rays, however, have so much energy that when they bounce off an atom, they knock electrons away from the atom. Scientists, therefore, cannot use X rays to take a picture of an atom without changing the atom. They must use a different method to get an accurate picture.

Electron microscopes provide scientists with an alternate method. Scientists shine electrons, instead of light, on an atom. As discussed in the Electrons as Waves section of this article, electrons have wavelike properties, so they can behave like light waves. The simplest type of electron microscope focuses the electrons reflected off of an object and translates the pattern formed by the reflected electrons into a visible display. Scientists have used this technique to create images of tiny insects and even individual living cells, but they have not been able to use it to make a clear image of objects smaller than about 10 nanometers (abbreviated nm), or 1 × 10-8 m (4 × 10-7 in).

To get to the level of individual atoms, scientists must use a more powerful type of electron microscope called a scanning tunneling microscope (STM). An STM uses a tiny probe, the tip of which can be as small as a single atom, to scan an object. An STM takes advantage of another wavelike property of electrons called tunneling. Tunneling allows electrons emitted from the probe of the microscope to penetrate, or tunnel into, the surface of the object being examined. The rate at which the electrons tunnel from the probe to the surface is related to the distance between the probe and the surface. These moving electrons generate a tiny electric current that the STM measures. The STM constantly adjusts the height of the probe to keep the current constant. By tracking how the height of the probe changes as the probe moves over the surface, scientists can get a detailed map of the surface. The map can be so detailed that individual atoms on the surface are visible.

Studying single atoms or small samples of atoms can help scientists understand atomic structure. However, all atoms, even atoms that are part of a solid material, are constantly in motion. This constant motion makes them difficult to examine. To study single atoms, scientists must slow the atoms down and confine them to one place. Scientists can slow and trap atoms using devices called particle traps.

Slowing down atoms is actually the same as cooling them. This is because an atom’s rate of motion is directly related to its temperature. Atoms that are moving very quickly cause a substance to have a high temperature. Atoms moving more slowly create a lower temperature. Scientists therefore build traps that cool atoms down to a very low temperature.

Several different types of particle traps exist. Some traps are designed to slow down ions, while others are designed to slow electrically neutral atoms. Traps for ions often use electric and magnetic fields to influence the movement of the particle, confining it in a small space or slowing it down. Traps for neutral atoms often use lasers, beams of light in which the light waves are uniform and consistent. Light has no mass, but it moves so quickly that it does have momentum. This property allows the light to affect other particles, or "bump" into them. When laser light collides with atoms, the momentum of the light forces the atoms to change speed and direction.

Scientists use trapped and cooled atoms for a variety of experiments, including those that precisely measure the properties of individual atoms and those in which scientists construct extremely accurate atomic clocks. Atomic clocks keep track of time by counting waves of radiation emitted by atoms in traps inside the clock. Because the traps hold the atoms at low temperatures, the mechanisms inside the clock can exercise more control over the atom, reducing the possibility of error. Scientists can also use isolated atoms to measure the force of gravity in an area with extreme accuracy. These measurements are useful in oil exploration, among other things. A deposit of oil or other substance beneath Earth’s surface has a different density than the material surrounding it. The strength of the pull of gravity in an area depends on the density of material in the area, so these changes in density produce changes in the local strength of gravity. Advances in the manipulation of atoms have also raised the possibility of using atoms to etch electronic circuits. This would help make the circuits smaller and thereby allow more circuits to fit in a tinier area.

In 1995 American physicists used particle traps to cool a sample of rubidium atoms to a temperature near absolute zero (-273°C, or -459°F). Absolute zero is the temperature at which all motion stops. When the scientists cooled the rubidium atoms to such a low temperature, the atoms slowed almost to a stop. The scientists knew that the momentum of the atoms, which is related to their speed, was close to zero. At this point, a special rule of quantum physics, called the uncertainty principle, greatly affected the positions of the atoms. This rule states that the momentum and position of a particle both cannot have precise values at the same time. The scientists had a fairly precise value for the atom’s momentum (nearly zero), so the positions of the atoms became very imprecise. The position of each atom could be described as a large, fuzzy cloud of probability. The atoms were very close together in the trap, so the probability clouds of many atoms overlapped one another. It was impossible for the scientists to tell where one atom ended and another began. In effect, the atoms formed one huge particle. This new state of matter is called a Bose-Einstein condensate.

Spectroscopy is the study of the radiation, or energy, that atoms, ions, molecules, and atomic nuclei emit. This emitted energy is usually in the form of electromagnetic radiation-vibrating electric and magnetic waves. Electromagnetic waves can have a variety of wavelengths, including those of visible light. X rays, ultraviolet radiation, and infrared radiation are also forms of electromagnetic radiation. Scientists use spectroscopes to measure this emitted radiation.

Atoms emit radiation when their electrons lose energy and drop down to lower orbitals, or energy states, as described in the Electron Energy Levels section above. The difference in energy between the orbitals determines the wavelength of the emitted radiation. This radiation can be in the form of visible light for outer electrons, or it can be radiation of shorter wavelengths, such as X-ray radiation, for inner electrons. Because the energies of the orbitals are strictly defined and differ from element to element, atoms of a particular element can only emit certain wavelengths of radiation. By studying the wavelengths of radiation emitted by a substance, scientists can identify the element or elements comprising the substance. For example, the outer electrons in a sodium atom emit a characteristic yellow light when they return to lower orbitals. This is why street lamps that use sodium vapor have a yellowish glow (See also Sodium-Vapor Lamp).

Chemists often use a procedure called a flame test to identify elements. In a flame test, the chemist burns a sample of the element. The heat excites the outer electrons in the element’s atoms, making the electrons jump to higher energy orbitals. When the electrons drop back down to their original orbitals, they emit light characteristic of that element. This light colors the flame and allows the chemist to identify the element.

The inner electrons of atoms also emit radiation that can help scientists identify elements. The energy it takes to boost an inner electron to a higher orbital is directly related to the positive charge of the nucleus and the pull this charge exerts on the electron. When the electron drops back to its original level, it emits the same amount of energy it absorbed, so the emitted energy is also related to the nucleus’s charge. The charge on the nucleus is equal to the atom’s atomic number.

Scientists measure the energy of the emitted radiation by measuring the radiation’s wavelength. The radiation’s energy is directly related to its wavelength, which usually resembles that of an X ray for the inner electrons. By measuring the wavelength of the radiation that an atom’s inner electron emits, scientists can identify the atom by its atomic number. Scientists used this method in the 1910s to identify the atomic number of the elements and to place the elements in their correct order in the periodic table. The method is still used today to identify particularly heavy elements (those with atomic numbers greater than 100) that are produced a few atoms at a time in large accelerators (see Transuranium Elements).

Atomic nuclei emit radiation when they undergo radioactive decay, as discussed in the Radioactivity section above. Nuclei usually emit radiation with very short wavelengths (and therefore high energy) when they decay. Often this radiation is in the form of gamma rays, a form of electromagnetic radiation with wavelengths even shorter than X rays. Once again, nuclei of different elements emit radiation of characteristic wavelengths. Scientists can identify nuclei by measuring this radiation. This method is especially useful in neutron activation analysis, a technique scientists use for identifying the presence of tiny amounts of elements. Scientists bombard samples that they wish to identify with neutrons. Some of the neutrons join the nuclei, making them radioactive. When the nuclei decay, they emit radiation that allows the scientists to identify the substance. Environmental scientists use neutron activation analysis in studying air and water pollution. Forensic scientists, who study evidence related to crimes, use this technique to identify gunshot residue and traces of poisons.

Particle accelerators are devices that increase the speed of a beam of elementary particles such as protons and electrons. Scientists use the accelerated beam to study collisions between particles. The beam can collide with a target of stationary particles, or it can collide with another accelerated beam of particles moving in the opposite direction. If physicists use the nucleus of an atom as the target, the particles and radiation produced in the collision can help them learn about the nucleus. The faster the particles move, the higher the energy they contain. If collisions occur at very high energy, it is possible to create particles never before detected. In certain circumstances, energy can be converted to matter, resulting in heavier particles after the collision.

Cyclotrons and linear accelerators are two of the most important kinds of particle accelerators. In a cyclotron, a magnetic field holds a beam of charged particles in a circular path. An electric field interacts with the particles’ electric charge to give them a boost of energy and speed each time the beam goes around. In linear accelerators, charged particles move in a straight line. They receive many small boosts of energy from electric fields as they move through the accelerator.

Bombarding nuclei with beams of neutrons forces the nuclei to absorb some of the neutrons and become unstable. The unstable nuclei then decay radioactively. The way atoms decay tells scientists about the original structure of the atom. Scientists can also deduce the size and shape of nuclei from the way particles scatter from nuclei when they collide. Another use of particle accelerators is to create new and exotic isotopes, including atoms of elements with very high atomic numbers that are not found in nature.

At higher energy levels, using particles moving at much higher speeds, scientists can use accelerators to look inside protons and neutrons to examine their internal structure. At these energy levels, accelerators can produce new types of particles. Some of these particles are similar to protons or neutrons but have larger masses and are very unstable. Others have a structure similar to the pion, the particle that is exchanged between the proton and neutron as part of the strong force that binds the nucleus together. By creating new particles and studying their properties, physicists have been able to deduce their common internal structure and to classify them using the theory of quarks. High-energy collisions between one particle and another often produce hundreds of particles. Experimenters have the challenging task of identifying and measuring all of these particles, some of which exist for only the tiniest fraction of a second.

Beginning with Democritus, who lived during the late 5th and early 4th centuries bc, Greek philosophers developed a theory of matter that was not based on experimental evidence, but on their attempts to understand the universe in philosophical terms. According to this theory, all matter was composed of tiny, indivisible particles called atoms (from the Greek word atomos, meaning "indivisible"). If a sample of a pure element was divided into smaller and smaller parts, eventually a point would be reached at which no further cutting would be possible-this was the atom of that element, the smallest possible bit of that element.

According to the ancient Greeks, atoms were all made of the same basic material, but atoms of different elements had different sizes and shapes. The sizes, shapes, and arrangements of a material’s atoms determined the material’s properties. For example, the atoms of a fluid were smooth so that they could easily slide over one another, while the atoms of a solid were rough and jagged so that they could attach to one another. Other than the atoms, matter was empty space. Atoms and empty space were believed to be the ultimate reality.

Although the notion of atoms as tiny bits of elemental matter is consistent with modern atomic theory, the researchers of prior eras did not understand the nature of atoms or their interactions in materials. For centuries scientists did not have the methods or technology to test their theories about the basic structure of matter, so people accepted the ancient Greek view.

The work of British chemist John Dalton at the beginning of the 19th century revealed some of the first clues about the true nature of atoms. Dalton studied how quantities of different elements, such as hydrogen and oxygen, could combine to make other substances, such as water. In his book A New System of Chemical Philosophy (1808), Dalton made two assertions about atoms: (1) atoms of each element are all identical to one another but different from the atoms of all other elements, and (2) atoms of different elements can combine to form more complex substances.

Dalton’s idea that different elements had different atoms was unlike the Greek idea of atoms. The characteristics of Dalton’s atoms determined the chemical and physical properties of a substance, no matter what the substance’s form. For example, carbon atoms can form both hard diamonds and soft graphite. In the Greek theory of atoms, diamond atoms would be very different from graphite atoms. In Dalton’s theory, diamond atoms would be very similar to graphite atoms because both substances are composed of the same chemical element.

While developing his theory of atoms, Dalton observed that two elements can combine in more than one way. For example, modern scientists know that carbon monoxide (CO) and carbon dioxide (CO2) are both compounds of carbon and oxygen. According to Dalton’s experiments, the quantities of an element needed to form different compounds are always whole-number multiples of one another. For example, two times as much oxygen is needed to form a liter of CO2 than is needed to form a liter of CO. Dalton correctly concluded that compounds were created when atoms of pure elements joined together in fixed proportions to form units that scientists today call molecules.

Scientists in the early 19th century struggled in another area of atomic theory. They tried to understand how atoms of a single element could exist in solid, liquid, and gaseous forms. Scientists correctly proposed that atoms in a solid attract each other with enough force to hold the solid together, but they did not understand why the atoms of liquids and gases did not attract each other as strongly. Some scientists theorized that the forces between atoms were attractive at short distances (such as when the atoms were packed very close together to form a solid) and repulsive at larger distances (such as in a gas, where the atoms are on the average relatively far apart).

Scientists had difficulty solving the problem of states of matter because they did not adequately understand the nature of heat. Today scientists recognize that heat is a form of energy, and that different amounts of this energy in a substance lead to different states of matter. In the 19th century, however, people believed that heat was a material substance, called caloric, that could be transferred from one object to another. This explanation of heat was called the caloric theory. Dalton used the caloric theory to propose that each molecule of a gas is surrounded by caloric, which exerts a repulsive force on other molecules. According to Dalton’s theory, as a gas is heated, more caloric is added to the gas, which increases the repulsive force between the molecules. More caloric would also cause the gas to exert a greater pressure on the walls of its container, in accordance with scientists’ experiments.

This early explanation of heat and states of matter broke down when experiments in the middle of the 19th century showed that heat could change into energy of motion. The laws of physics state that the amount of energy in a system cannot increase, so scientists had to accept that heat must be energy, not a substance. This revelation required a new theory of how atoms in different states of matter behave.

In the early 19th century Italian chemist Amedeo Avogadro made an important advance in the understanding of how atoms and molecules in a gas behave. Avogadro began his work from a theory developed by Dalton. Dalton’s theory proposed that a gaseous compound, formed by combining equal numbers of atoms of two elements, should have the same number of molecules as the atoms in one of the original elements. For example, ten atoms of the element hydrogen (H) combine with ten atoms of chlorine (Cl) to form ten gaseous hydrogen chloride (HCl) molecules.

In 1811 Avogadro developed a law of physics that seemed to contradict Dalton’s theory. Avogadro’s law states that equal volumes of different gases contain the same number of particles (atoms or molecules) if both gases are at the same temperature and pressure. In Dalton’s experiment, the volume of the original vessels containing the hydrogen or chlorine gases was the same as the volume of the vessel containing the hydrogen chloride gas. The pressures of the original hydrogen and chlorine gases were equal, but the pressure of the hydrochloric gas was twice as great as either of the original gases. According to Avogadro’s law, this doubled pressure would mean that there were twice as many hydrogen chloride gas particles than there had been chlorine particles prior to their combination.

To reconcile the results of Dalton’s experiment with his new rule, Avogadro was forced to conclude that the original vessels of hydrogen or chlorine contained only half as many particles as Dalton had thought. Dalton, however, knew the total weight of each gas in the vessels, as well as the weight of an individual atom of each gas, so he knew the total number of atoms of each gas that was present in the vessels. Avogadro reconciled the fact that there were twice as many atoms as there were particles in the vessels by proposing that gases such as hydrogen and chlorine are really made up of molecules of hydrogen and chlorine, with two atoms in each molecule. Today scientists write the chemical symbols for hydrogen and chlorine as H2 and Cl2, respectively, indicating that there are two atoms in each molecule. One molecule of hydrogen and one molecule of chlorine combine to form two molecules of hydrogen chlorine (H2 + Cl2 → 2HCl). The sample of hydrogen chloride contains twice the number of particles as either the hydrogen or chlorine because two molecules of hydrogen chloride form when a molecule of hydrogen combines with a molecule of chlorine.

The work of Dalton and Avogadro led to a consistent view of the quantities of different gases that could be combined to form compounds, but scientists still did not understand the nature of the forces that attracted the atoms to one another in compounds and molecules. Scientists suspected that electrical forces might have something to do with that attraction, but they found it difficult to understand how electrical forces could allow two identical, neutral hydrogen atoms to attract one another to form a hydrogen molecule.

In the 1830s, British physicist Michael Faraday took the first significant step toward appreciating the importance of electrical forces in compounds. Faraday placed two electrodes connected to opposite terminals of a battery into a solution of water containing a dissolved compound. As the electric current flowed through the solution, Faraday observed that one of the elements that comprised the dissolved compound became deposited on one electrode while the other element became deposited on the other electrode. The electric current provided by the electrodes undid the coupling of atoms in the compound. Faraday also observed that the quantity of each element deposited on an electrode was directly proportional to the total quantity of current that flowed through the solution-the stronger the current, the more material became deposited on the electrode. This discovery made it clear that electrical forces must be in some way responsible for the joining of atoms in compounds.

Despite these significant discoveries, most scientists did not immediately accept that atoms as described by Dalton, Faraday, and Avogadro were responsible for the chemical and physical behavior of substances. Before the end of the 19th century, many scientists believed that all chemical and physical properties could be determined by the rules of heat, an understanding of atoms closer to that of the Greek philosophers. The development of the science of thermodynamics (the scientific study of heat) and the recognition that heat was a form of energy eliminated the role of caloric in atomic theory and made atomic theory more acceptable. The new theory of heat, called the kinetic theory, said that the atoms or molecules of a substance move faster, or gain kinetic energy, as heat energy is added to the substance. Nevertheless, a small but powerful group of scientists still did not accept the existence of atoms-they regarded atoms as convenient mathematical devices that explained the chemistry of compounds, not as real entities.

In 1905 French chemist Jean-Baptiste Perrin performed the final experiments that helped prove the atomic theory of matter. Perrin observed the irregular wiggling of pollen grains suspended in a liquid (a phenomenon called Brownian motion) and correctly explained that the wiggling was the result of atoms of the fluid colliding with the pollen grains. This experiment showed that the idea that materials were composed of real atoms in thermal motion was in fact correct.

As scientists began to accept atomic theory, researchers turned their efforts to understanding the electrical properties of the atom. Several scientists, most notably British scientist Sir William Crookes, studied the effects of sending electric current through a gas. The scientists placed a very small amount of gas in a sealed glass tube. The tube had electrodes at either end. When an electric current was applied to the gas, a stream of electrically charged particles flowed from one of the electrodes. This electrode was called the cathode, and the particles were called cathode rays.

At first scientists believed that the rays were composed of charged atoms or molecules, but experiments showed that the cathode rays could penetrate thin sheets of material, which would not be possible for a particle as large as an atom or a molecule. British physicist Sir Joseph John Thomson measured the velocity of the cathode rays and showed that they were much too fast to be atoms or molecules. No known force could accelerate a particle as heavy as an atom or a molecule to such a high speed. Thomson also measured the ratio of the charge of a cathode ray to the mass of the cathode ray. The value he measured was about 1,000 times larger than any previous measurement associated with charged atoms or molecules, indicating that within cathode rays particularly tiny masses carried relatively large amounts of charge. Thomson studied different gases and always found the same value for the charge-to-mass ratio. He concluded that he was observing a new type of particle, which carried a negative electric charge but was about a thousand times less massive than the lightest known atom. He also concluded that these particles were constituents of all atoms. Today scientists know these particles as electrons, and Thomson is credited with their discovery.

Scientists realized that if all atoms contain electrons but are electrically neutral, atoms must also contain an equal quantity of positive charge to balance the electrons’ negative charge. Furthermore, if electrons are indeed much less massive than even the lightest atom, then this positive charge must account for most of the mass of the atom. Thomson proposed a model by which this phenomenon could occur: He suggested that the atom was a sphere of positive charge into which the negative electrons were imbedded, like raisins in a loaf of raisin bread. In 1911 British scientist Ernest Rutherford set out to test Thomson’s proposal by firing a beam of charged particles at atoms.

Rutherford chose alpha particles for his beam. Alpha particles are heavy particles with twice the positive charge of a proton. Alpha particles are now known to be the nuclei of helium atoms, which contain two protons and two neutrons. If Thomson’s model of the atom was correct, Rutherford theorized that the electric charge and the mass of the atoms would be too spread out to significantly deflect the alpha particles. Rutherford was quite surprised to observe something very different. Most of the alpha particles did indeed change their paths by a small angle, and occasionally an alpha particle bounced back in the opposite direction. The alpha particles that bounced back must have struck something at least as heavy as themselves. This led Rutherford to propose a very different model for the atom. Instead of supposing that the positive charge and mass were spread throughout the volume of the atom, he theorized that it was concentrated in the center of the atom. Rutherford called this concentrated region of electric charge the nucleus of the atom.

In the span of 100 years, from Dalton to Rutherford, the basic ideas of atomic structure evolved from very primitive concepts of how atoms combined with one another to an understanding of the constituents of atoms-a positively charged nucleus surrounded by negatively charged electrons. The interactions between the nucleus and the electrons still required study. It was natural for physicists to model the atom, in which tiny electrons orbit a much more massive nucleus, after a familiar structure such as the solar system, in which planets orbit around a much more massive Sun. Rutherford’s model of the atom did indeed resemble a tiny solar system. The only difference between early models of the nuclear atom and the solar system was that atoms were held together by electromagnetic force, while gravitational force holds together the solar system.

Danish physicist Niels Bohr used new knowledge about the radiation emitted from atoms to develop a model of the atom significantly different from Rutherford’s model. Scientists of the 19th century discovered that when an electrical discharge passes through a small quantity of a gas in a glass tube, the atoms in the gas emit light. This radiation occurs only at certain discrete wavelengths, and different elements and compounds emit different wavelengths. Bohr, working in Rutherford’s laboratory, set out to understand the emission of radiation at these wavelengths based on the nuclear model of the atom.

Using Rutherford’s model of the atom as a miniature solar system, Bohr developed a theory by which he could predict the same wavelengths scientists had measured radiating from atoms with a single electron. However, when conceiving this theory, Bohr was forced to make some startling conclusions. He concluded that because atoms emit light only at discrete wavelengths, electrons could only orbit at certain designated radii, and light could be emitted only when an electron jumped from one of these designated orbits to another. Both of these conclusions were in disagreement with classical physics, which imposed no strict rules on the size of orbits. To make his theory work, Bohr had to propose special rules that violated the rules of classical physics. He concluded that, on the atomic scale, certain preferred states of motion were especially stable. In these states of motion an orbiting electron (contrary to the laws of electromagnetism) would not radiate energy.

At the same time that Bohr and Rutherford were developing the nuclear model of the atom, other experiments indicated similar failures of classical physics. These experiments included the emission of radiation from hot, glowing objects (called thermal radiation) and the release of electrons from metal surfaces illuminated with ultraviolet light (the photoelectric effect). Classical physics could not account for these observations, and scientists began to realize that they needed to take a new approach. They called this new approach quantum mechanics (see Quantum Theory), and they developed a mathematical basis for it in the 1920s. The laws of classical physics work perfectly well on the scale of everyday objects, but on the tiny atomic scale, the laws of quantum mechanics apply.

The quantum mechanical view of atomic structure maintains some of Rutherford and Bohr’s ideas. The nucleus is still at the center of the atom and provides the electrical attraction that binds the electrons to the atom. Contrary to Bohr’s theory, however, the electrons do not circulate in definite planet-like orbits. The quantum-mechanical approach acknowledges the wavelike character of electrons and provides the framework for viewing the electrons as fuzzy clouds of negative charge. Electrons still have assigned states of motion, but these states of motion do not correspond to fixed orbits. Instead, they tell us something about the geometry of the electron cloud-its size and shape and whether it is spherical or bunched in lobes like a figure eight. Physicists called these states of motion orbitals. Quantum mechanics also provides the mathematical basis for understanding how atoms that join together in molecules share electrons. Nearly 100 years after Faraday’s pioneering experiments, the quantum theory confirmed that it is indeed electrical forces that are responsible for the structure of molecules.

Two of the rules of quantum theory that are most important to explaining the atom are the idea of wave-particle duality and the exclusion principle. French physicist Louis de Broglie first suggested that particles could be described as waves in 1924. In the same decade, Austrian physicist Erwin Schrödinger and German physicist Werner Heisenberg expanded de Broglie’s ideas into formal, mathematical descriptions of quantum mechanics. The exclusion principle was developed by Austrian-born American physicist Wolfgang Pauli in 1925. The Pauli exclusion principle states that no two electrons in an atom can have exactly the same characteristics.

The combination of wave-particle duality and the Pauli exclusion principle sets up the rules for filling electron orbitals in atoms. The way electrons fill up orbitals determines the number of electrons that end up in the atom’s valence shell. This in turn determines an atom’s chemical and physical properties, such as how it reacts with other atoms and how well it conducts electricity. These rules explain why atoms with similar numbers of electrons can have very different properties, and why chemical properties reappear again and again in a regular pattern among the elements.

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Nuclear Energy

Nuclear Energy, energy released during the splitting or fusing of atomic nuclei. The energy of any system, whether physical, chemical, or nuclear, is manifested by the system’s ability to do work or to release heat or radiation. The total energy in a system is always conserved, but it can be transferred to another system or changed in form.

Until about 1800 the principal fuel was wood, its energy derived from solar energy stored in plants during their lifetimes. Since the Industrial Revolution, people have depended on fossil fuels-coal, petroleum, and natural gas-also derived from stored solar energy. When a fossil fuel such as coal is burned, atoms of hydrogen and carbon in the coal combine with oxygen atoms in air. Water and carbon dioxide are produced and heat is released, equivalent to about 1.6 kilowatt-hours per kilogram or about 10 electron volts (eV) per atom of carbon. This amount of energy is typical of chemical reactions resulting from changes in the electronic structure of the atoms. A part of the energy released as heat keeps the adjacent fuel hot enough to keep the reaction going.

The atom consists of a small, massive, positively charged core (nucleus) surrounded by electrons (see Atom). The nucleus, containing most of the mass of the atom, is itself composed of neutrons and protons bound together by very strong nuclear forces, much greater than the electrical forces that bind the electrons to the nucleus. The mass number A of a nucleus is the number of nucleons, or protons and neutrons, it contains; the atomic number Z is the number of positively charged protons. A specific nucleus is designated as ¿U the expression ¯U, for example, represents uranium-235. See Isotope.

The binding energy of a nucleus is a measure of how tightly its protons and neutrons are held together by the nuclear forces. The binding energy per nucleon, the energy required to remove one neutron or proton from a nucleus, is a function of the mass number A. The curve of binding energy implies that if two light nuclei near the left end of the curve coalesce to form a heavier nucleus, or if a heavy nucleus at the far right splits into two lighter ones, more tightly bound nuclei result, and energy will be released.

Nuclear energy, measured in millions of electron volts (MeV), is released by the fusion of two light nuclei, as when two heavy hydrogen nuclei, deuterons (ªH), combine in the reaction

producing a helium-3 atom, a free neutron (¦n), and 3.2 MeV, or 5.1 × 10-13 J (1.2 × 10-13 cal). Nuclear energy is also released when the fission of a heavy nucleus such as ¯U is induced by the absorption of a neutron as in

producing cesium-140, rubidium-93, three neutrons, and 200 MeV, or 3.2 × 10-11 J (7.7 × 10-12 cal). A nuclear fission reaction releases 10 million times as much energy as is released in a typical chemical reaction. See Nuclear Chemistry.

The two key characteristics of nuclear fission important for the practical release of nuclear energy are both evident in equation (2). First, the energy per fission is very large. In practical units, the fission of 1 kg (2.2 lb) of uranium-235 releases 18.7 million kilowatt-hours as heat. Second, the fission process initiated by the absorption of one neutron in uranium-235 releases about 2.5 neutrons, on the average, from the split nuclei. The neutrons released in this manner quickly cause the fission of two more atoms, thereby releasing four or more additional neutrons and initiating a self-sustaining series of nuclear fissions, or a chain reaction, which results in continuous release of nuclear energy.

Naturally occurring uranium contains only 0.71 percent uranium-235; the remainder is the nonfissile isotope uranium-238. A mass of natural uranium by itself, no matter how large, cannot sustain a chain reaction because only the uranium-235 is easily fissionable. The probability that a fission neutron with an initial energy of about 1 MeV will induce fission is rather low, but the probability can be increased by a factor of hundreds when the neutron is slowed down through a series of elastic collisions with light nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for the design of practical energy-producing fission reactors.

In December 1942 at the University of Chicago, the Italian physicist Enrico Fermi succeeded in producing the first nuclear chain reaction. This was done with an arrangement of natural uranium lumps distributed within a large stack of pure graphite, a form of carbon. In Fermi's "pile," or nuclear reactor, the graphite moderator served to slow the neutrons.

The first large-scale nuclear reactors were built in 1944 at Hanford, Washington, for the production of nuclear weapons material. The fuel was natural uranium metal; the moderator, graphite. Plutonium was produced in these plants by neutron absorption in uranium-238; the power produced was not used.

A variety of reactor types, characterized by the type of fuel, moderator, and coolant used, have been built throughout the world for the production of electric power. In the United States, with few exceptions, power reactors use nuclear fuel in the form of uranium oxide isotopically enriched to about three percent uranium-235. The moderator and coolant are highly purified ordinary water. A reactor of this type is called a light-water reactor (LWR).

In the pressurized-water reactor (PWR), a version of the LWR system, the water coolant operates at a pressure of about 150 atmospheres. It is pumped through the reactor core, where it is heated to about 325° C (about 620° F). The superheated water is pumped through a steam generator, where, through heat exchangers, a secondary loop of water is heated and converted to steam. This steam drives one or more turbine generators, is condensed, and is pumped back to the steam generator. The secondary loop is isolated from the water in the reactor core and, therefore, is not radioactive. A third stream of water from a lake, river, or cooling tower is used to condense the steam. The reactor pressure vessel is about 15 m (about 49 ft) high and 5 m (about 16.4 ft) in diameter, with walls 25 cm (about 10 in) thick. The core houses some 82 metric tons of uranium oxide contained in thin corrosion-resistant tubes clustered into fuel bundles.

In the boiling-water reactor (BWR), a second type of LWR, the water coolant is permitted to boil within the core, by operating at somewhat lower pressure. The steam produced in the reactor pressure vessel is piped directly to the turbine generator, is condensed, and is then pumped back to the reactor. Although the steam is radioactive, there is no intermediate heat exchanger between the reactor and turbine to decrease efficiency. As in the PWR, the condenser cooling water has a separate source, such as a lake or river.

The power level of an operating reactor is monitored by a variety of thermal, flow, and nuclear instruments. Power output is controlled by inserting or removing from the core a group of neutron-absorbing control rods. The position of these rods determines the power level at which the chain reaction is just self-sustaining.

During operation, and even after shutdown, a large, 1,000-megawatt (MW) power reactor contains billions of curies of radioactivity. Radiation emitted from the reactor during operation and from the fission products after shutdown is absorbed in thick concrete shields around the reactor and primary coolant system. Other safety features include emergency core cooling systems to prevent core overheating in the event of malfunction of the main coolant systems and, in most countries, a large steel and concrete containment building to retain any radioactive elements that might escape in the event of a leak.

Although more than 100 nuclear power plants were operating or being built in the United States at the beginning of the 1980s, in the aftermath of the Three Mile Island accident in Pennsylvania in 1979 safety concerns and economic factors combined to block any additional growth in nuclear power. No orders for nuclear plants have been placed in the United States since 1978, and some plants that have been completed have not been allowed to operate. In 1996 about 22 percent of the electric power generated in the United States came from nuclear power plants. In contrast, in France almost three-quarters of the electricity generated was from nuclear power plants.

In the initial period of nuclear power development in the early 1950s, enriched uranium was available only in the United States and the Union of Soviet Socialist Republics (USSR). The nuclear power programs in Canada, France, and the United Kingdom therefore centered about natural uranium reactors, in which ordinary water cannot be used as the moderator because it absorbs too many neutrons. This limitation led Canadian engineers to develop a reactor cooled and moderated by deuterium oxide (D2O), or heavy water. The Canadian deuterium-uranium reactor known as CANDU has operated satisfactorily in Canada, and similar plants have been built in India, Argentina, and elsewhere.

In the United Kingdom and France the first full-scale power reactors were fueled with natural uranium metal, were graphite-moderated, and were cooled with carbon dioxide gas under pressure. These initial designs have been superseded in the United Kingdom by a system that uses enriched uranium fuel. In France the initial reactor type chosen was dropped in favor of the PWR of U.S. design when enriched uranium became available from French isotope-enrichment plants. Russia and the other successor states of the USSR had a large nuclear power program, using both graphite-moderated and PWR systems.

Nuclear power plants similar to the PWR are used for the propulsion plants of large surface naval vessels such as the aircraft carrier USS Nimitz. The basic technology of the PWR system was first developed in the U.S. naval reactor program directed by Admiral Hyman G. Rickover. Reactors for submarine propulsion are generally physically smaller and use more highly enriched uranium to permit a compact core. The United States, the United Kingdom, Russia, and France all have nuclear-powered submarines with such power plants.

Three experimental seagoing nuclear cargo ships were operated for limited periods by the United States, Germany, and Japan. Although they were technically successful, economic conditions and restrictive port regulations brought an end to these projects. The Soviet government built the first successful nuclear-powered icebreaker, Lenin, for use in clearing the Arctic sea-lanes.

A variety of small nuclear reactors have been built in many countries for use in education and training, research, and the production of radioactive isotopes. These reactors generally operate at power levels near one MW, and they are more easily started up and shut down than larger power reactors.

A widely used type is called the swimming-pool reactor. The core is partially or fully enriched uranium-235 contained in aluminum alloy plates, immersed in a large pool of water that serves as both coolant and moderator. Materials may be placed directly in or near the reactor core to be irradiated with neutrons. Various radioactive isotopes can be produced for use in medicine, research, and industry (see Isotopic Tracer). Neutrons may also be extracted from the reactor core by means of beam tubes to be used for experimentation.

Uranium, the natural resource on which nuclear power is based, occurs in scattered deposits throughout the world. Its total supply is not fully known, and may be limited unless sources of very low concentration such as granites and shale were to be used. Conservatively estimated U.S. resources of uranium having an acceptable cost lie in the range of two million to five million metric tons. The lower amount could support an LWR nuclear power system providing about 30 percent of U.S. electric power for only about 50 years. The principal reason for this relatively brief life span of the LWR nuclear power system is its very low efficiency in the use of uranium: only approximately one percent of the energy content of the uranium is made available in this system.

The key feature of a breeder reactor is that it produces more fuel than it consumes. It does this by promoting the absorption of excess neutrons in a fertile material. Several breeder reactor systems are technically feasible. The breeder system that has received the greatest worldwide attention uses uranium-238 as the fertile material. When uranium-238 absorbs neutrons in the reactor, it is transmuted to a new fissionable material, plutonium, through a nuclear process called b (beta) decay. The sequence of nuclear reactions is

In beta decay a nuclear neutron decays into a proton and a beta particle (a high-energy electron).

When plutonium-239 itself absorbs a neutron, fission can occur, and on the average about 2.8 neutrons are released. In an operating reactor, one of these neutrons is needed to cause the next fission and keep the chain reaction going. On the average about 0.5 neutron is uselessly lost by absorption in the reactor structure or coolant. The remaining 1.3 neutrons can be absorbed in uranium-238 to produce more plutonium via the reactions in equation (3).

The breeder system that has had the greatest development effort is called the liquid-metal fast breeder reactor (LMFBR). In order to maximize the production of plutonium-239, the velocity of the neutrons causing fission must remain fast-at or near their initial release energy. Any moderating materials, such as water, that might slow the neutrons must be excluded from the reactor. A molten metal, liquid sodium, is the preferred coolant liquid. Sodium has very good heat transfer properties, melts at about 100° C (about 212° F), and does not boil until about 900° C (about 1650° F). Its main drawbacks are its chemical reactivity with air and water and the high level of radioactivity induced in it in the reactor.

Development of the LMFBR system began in the United States before 1950, with the construction of the first experimental breeder reactor, EBR-1. A larger U.S. program, on the Clinch River, was halted in 1983, and only experimental work was to continue (see Tennessee Valley Authority). In the United Kingdom, France, and Russia and the other successor states of the USSR, working breeder reactors were installed, and experimental work continued in Germany and Japan.

In one design of a large LMFBR power plant, the core of the reactor consists of thousands of thin stainless steel tubes containing mixed uranium and plutonium oxide fuel: about 15 to 20 percent plutonium-239, the remainder uranium. Surrounding the core is a region called the breeder blanket, which contains similar rods filled only with uranium oxide. The entire core and blanket assembly measures about 3 m (about 10 ft) high by about 5 m (about 16.4 ft) in diameter and is supported in a large vessel containing molten sodium that leaves the reactor at about 500° C (about 930° F). This vessel also contains the pumps and heat exchangers that aid in removing heat from the core. Steam is produced in a second sodium loop, separated from the radioactive reactor coolant loop by the intermediate heat exchangers in the reactor vessel. The entire nuclear reactor system is housed in a large steel and concrete containment building.

The first large-scale plant of this type for the generation of electricity, called Super-Phénix, went into operation in France in 1984. (However, concerns about operational safety and environmental contamination led the French government to announce in 1998 that Super-Phénix would be dismantled). An intermediate-scale plant, the BN-600, was built on the shore of the Caspian Sea for the production of power and the desalination of water. The British have a large 250-MW prototype in Scotland.

The LMFBR produces about 20 percent more fuel than it consumes. In a large power reactor enough excess new fuel is produced over 20 years to permit the loading of another similar reactor. In the LMFBR system about 75 percent of the energy content of natural uranium is made available, in contrast to the one percent in the LWR.

The hazardous fuels used in nuclear reactors present handling problems in their use. This is particularly true of the spent fuels, which must be stored or disposed of in some way.

Any electric power generating plant is only one part of a total energy cycle. The uranium fuel cycle that is employed for LWR systems currently dominates worldwide nuclear power production and includes many steps. Uranium, which contains about 0.7 percent uranium-235, is obtained from either surface or underground mines. The ore is concentrated by milling and then shipped to a conversion plant, where its elemental form is changed to uranium hexafluoride gas (UF6). At an isotope enrichment plant, the gas is forced against a porous barrier that permits the lighter uranium-235 to penetrate more readily than uranium-238. This process enriches uranium to about 3 percent uranium-235. The depleted uranium-the tailings-contain about 0.3 percent uranium-235. The enriched product is sent to a fuel fabrication plant, where the UF6 gas is converted to uranium oxide powder, then into ceramic pellets that are loaded into corrosion-resistant fuel rods. These are assembled into fuel elements and are shipped to the reactor power plant. The world’s supply of enriched uranium fuel for powering commercial nuclear power plants is produced by five consortiums located in the United States, Western Europe, Russia, and Japan. The United States consortium-the federally owned United States Enrichment Corporation-produces 40 percent of this enriched uranium.

A typical 1,000-MW pressurized-water reactor has about 200 fuel elements, one-third of which are replaced each year because of the depletion of the uranium-235 and the buildup of fission products that absorb neutrons. At the end of its life in the reactor, the fuel is tremendously radioactive because of the fission products it contains and hence is still producing a considerable amount of energy. The discharged fuel is placed in water storage pools at the reactor site for a year or more.

At the end of the cooling period the spent fuel elements are shipped in heavily shielded casks either to permanent storage facilities or to a chemical reprocessing plant. At a reprocessing plant, the unused uranium and the plutonium-239 produced in the reactor are recovered and the radioactive wastes concentrated. (In the late 1990s neither such facility was yet available in the United States for power plant fuel, and temporary storage was used.)

The spent fuel still contains almost all the original uranium-238, about one-third of the uranium-235, and some of the plutonium-239 produced in the reactor. In cases where the spent fuel is sent to permanent storage, none of this potential energy content is used. In cases where the fuel is reprocessed, the uranium is recycled through the diffusion plant, and the recovered plutonium-239 may be used in place of some uranium-235 in new fuel elements. At the end of the 20th century, no reprocessing of fuel occurred in the United States because of environmental, health, and safety concerns, and the concern that plutonium-239 could be used illegally for the manufacture of weapons.

In the fuel cycle for the LMFBR, plutonium bred in the reactor is always recycled for use in new fuel. The feed to the fuel-element fabrication plant consists of recycled uranium-238, depleted uranium from the isotope separation plant stockpile, and part of the recovered plutonium-239. No additional uranium needs to be mined, as the existing stockpile could support many breeder reactors for centuries. Because the breeder produces more plutonium-239 than it requires for its own refueling, about 20 percent of the recovered plutonium is stored for later use in starting up new breeders. Because new fuel is bred from the uranium-238, instead of using only the natural uranium-235 content, about 75 percent of the potential energy of uranium is made available with the breeder cycle.

The final step in any of the fuel cycles is the long-term storage of the highly radioactive wastes, which remain biologically hazardous for thousands of years. Fuel elements may be stored in shielded, guarded repositories for later disposition or may be converted to very stable compounds, fixed in ceramics or glass, encapsulated in stainless steel canisters, and buried far underground in very stable geologic formations. However, the safety of such repositories is the subject of public controversy, especially in the geographic region in which the repository is located or is proposed to be built. For example, environmentalists plan to file a lawsuit to close a repository built near Carlsbad, New Mexico. In 1999, this repository began receiving shipments of radioactive waste from the manufacture of nuclear weapons in United States during the Cold War. Another controversy centers around a proposed repository at Yucca Mountain, Nevada. Opposition from state residents and questions about the geologic stability of this site have helped prolong government studies. Even if opened, the site will not receive shipments of radioactive waste until at least 2010 (see Nuclear Fuels and Wastes, Waste Management section below).

Public concern about the acceptability of nuclear power from fission arises from two basic features of the system. The first is the high level of radioactivity present at various stages of the nuclear cycle, including disposal. The second is the fact that the nuclear fuels uranium-235 and plutonium-239 are the materials from which nuclear weapons are made. See Nuclear Weapons; Radioactive Fallout.

U.S. President Dwight D. Eisenhower announced the U.S. Atoms for Peace program in 1953. It was perceived as offering a future of cheap, plentiful energy. The utility industry hoped that nuclear power would replace increasingly scarce fossil fuels and lower the cost of electricity. Groups concerned with conserving natural resources foresaw a reduction in air pollution and strip mining. The public in general looked favorably on this new energy source, seeing the program as a realization of hopes for the transition of nuclear power from wartime to peaceful uses.

Nevertheless, after this initial euphoria, reservations about nuclear energy grew as greater scrutiny was given to issues of nuclear safety and weapons proliferation. In the United States and other countries many groups oppose nuclear power. In addition, high construction costs, strict building and operating regulations, and high costs for waste disposal make nuclear power plants much more expensive to build and operate than plants that burn fossil fuels. In some industrialized countries, the nuclear power industry has come under growing pressure to cut operating expenses and become more cost-competitive. Other countries have begun or planned to phase out nuclear power completely.

At the end of the 20th century, many experts viewed Asia as the only possible growth area for nuclear power. In the late 1990s, China, Japan, South Korea, and Taiwan had nuclear power plants under construction. However, many European nations were reducing or reversing their commitments to nuclear power. For example, Sweden committed to phasing out nuclear power by 2010. France canceled several planned reactors and was considering the replacement of aging nuclear plants with environmentally safer fossil-fuel plants. Germany announced plans in 1998 to phase out nuclear energy. In the United States, no new reactors had been ordered since 1978.

In 1996, 21.9 percent of the electricity generated in the United States was produced by nuclear power. By 1998 that amount had decreased to 20 percent. Because no orders for nuclear plants have been placed since 1978, this share should continue to decline as existing nuclear plants are eventually closed. In 1998 Commonwealth Edison, the largest private owner and operator of nuclear plants in the United States, had only four of 12 nuclear power plants online. Industry experts cite economic, safety, and labor problems as reasons for these shutdowns.

Radioactive materials emit penetrating, ionizing radiation that can injure living tissues. The commonly used unit of radiation dose equivalent in humans is the sievert. (In the United States, rems are still used as a measure of dose equivalent. One rem equals 0.01 sievert.) Each individual in the United States and Canada is exposed to about 0.003 sievert per year from natural background radiation sources. An exposure to an individual of five sieverts is likely to be fatal. A large population exposed to low levels of radiation will experience about one additional cancer for each 10 sieverts total dose equivalent. See Radiation Effects, Biological.

Radiological hazards can arise in most steps of the nuclear fuel cycle. Radioactive radon gas is a colorless gas produced from the decay of uranium. As a result, radon is a common air pollutant in underground uranium mines. The mining and ore-milling operations leave large amounts of waste material on the ground that still contain small concentrations of uranium. To prevent the release of radioactive radon gas into the air from this uranium waste, these wastes must be stored in waterproof basins and covered with a thick layer of soil.

Uranium enrichment and fuel fabrication plants contain large quantities of three-percent uranium-235, in the form of corrosive gas, uranium hexafluoride, UF6. The radiological hazard, however, is low, and the usual care taken with a valuable material posing a typical chemical hazard suffices to ensure safety.

The safety of the power reactor itself has received the greatest attention. In an operating reactor, the fuel elements contain by far the largest fraction of the total radioactive inventory. A number of barriers prevent fission products from leaking into the air during normal operation. The fuel is clad in corrosion-resistant tubing. The heavy steel walls of the primary coolant system of the PWR form a second barrier. The water coolant itself absorbs some of the biologically important radioactive isotopes such as iodine. The steel and concrete building is a third barrier.

During the operation of a power reactor, some radioactive compounds are unavoidably released. The total exposure to people living nearby is usually only a few percent of the natural background radiation. Major concerns arise, however, from radioactive releases caused by accidents in which fuel damage occurs and safety devices fail. The major danger to the integrity of the fuel is a loss-of-coolant accident in which the fuel is damaged or even melts. Fission products are released into the coolant, and if the coolant system is breached, fission products enter the reactor building.

Reactor systems rely on elaborate instrumentation to monitor their condition and to control the safety systems used to shut down the reactor under abnormal circumstances. Backup safety systems that inject boron into the coolant to absorb neutrons and stop the chain reaction to further assure shutdown are part of the PWR design. Light-water reactor plants operate at high coolant pressure. In the event of a large pipe break, much of the coolant would flash into steam and core cooling could be lost. To prevent a total loss of core cooling, reactors are provided with emergency core cooling systems that begin to operate automatically on the loss of primary coolant pressure. In the event of a steam leak into the containment building from a broken primary coolant line, spray coolers are actuated to condense the steam and prevent a hazardous pressure rise in the building.

Despite the many safety features described above, an accident did occur in 1979 at the Three Mile Island PWR near Harrisburg, Pennsylvania. A maintenance error and a defective valve led to a loss-of-coolant accident. The reactor itself was shut down by its safety system when the accident began, and the emergency core cooling system began operating as required a short time into the accident. Then, however, as a result of human error, the emergency cooling system was shut off, causing severe core damage and the release of volatile fission products from the reactor vessel. Although only a small amount of radioactive gas escaped from the containment building, causing a slight rise in individual human exposure levels, the financial damage to the utility was very large, $1 billion or more, and the psychological stress on the public, especially those people who live in the area near the nuclear power plant, was in some instances severe.

The official investigation of the accident named operational error and inadequate control room design, rather than simple equipment failure, as the principal causes of the accident. It led to enactment of legislation requiring the Nuclear Regulatory Commission to adopt far more stringent standards for the design and construction of nuclear power plants. The legislation also required utility companies to assume responsibility for helping state and county governments prepare emergency response plans to protect the public health in the event of other such accidents.

Since 1981, the financial burdens imposed by these requirements have made it difficult to build and operate new nuclear power plants. Combined with other factors, such as high capital costs and long construction periods (which means builders must borrow more money and wait longer periods before earning a return on their investment), safety regulations have forced utility companies in the states of Washington, Ohio, Indiana, and New York to abandon partly completed plants after spending billions of dollars on them.

On April 26, 1986, another serious incident alarmed the world. One of four nuclear reactors at Chernobyl', near Pripyat’, about 130 km (about 80 mi) north of Kyiv (now in Ukraine) in the USSR, exploded and burned. Radioactive material spread over Scandinavia and northern Europe, as discovered by Swedish observers on April 28. According to the official report issued in August, the accident was caused by unauthorized testing of the reactor by its operators. The reactor went out of control; there were two explosions, the top of the reactor blew off, and the core was ignited, burning at temperatures of 1500° C (2800° F). Radiation about 50 times higher than that at Three Mile Island exposed people nearest the reactor, and a cloud of radioactive fallout spread westward. Unlike most reactors in western countries, including the United States, the reactor at Chernobyl' did not have a containment building. Such a structure could have prevented material from leaving the reactor site. About 135,000 people were evacuated, and more than 30 died. The plant was encased in concrete. By 1988, however, the other three Chernobyl' reactors were back in operation. One of the three remaining reactors was shut down in 1991 because of a fire in the reactor building. In 1994 Western nations developed a financial aid package to help close the entire plant, and a year later the Ukrainian government finally agreed to a plan that would shut down the remaining reactors by the year 2000.

The fuel reprocessing step poses a combination of radiological hazards. One is the accidental release of fission products if a leak should occur in chemical equipment or the cells and building housing it. Another may be the routine release of low levels of inert radioactive gases such as xenon and krypton. In 1966 a commercial reprocessing plant opened in West Valley, New York. But in 1972 this reprocessing plant was closed after generating more than 600,000 gallons of high-level radioactive waste. After the plant was closed, a portion of this radioactive waste was partially treated and cemented into nearly 20,000 steel drums. In 1996, the United States Department of Energy began to solidify the remaining liquid radioactive wastes into glass cylinders. At the end of the 20th century, no reprocessing plants were licensed in the United States.

Of major concern in chemical reprocessing is the separation of plutonium-239, a material that can be used to make nuclear weapons. The hazards of theft of plutonium-239, or its use for intentional but hidden production for weapons purposes, can best be controlled by political rather than technical means. Improved security measures at sensitive points in the fuel cycle and expanded international inspection by the International Atomic Energy Agency (IAEA) offer the best prospects for controlling the hazards of plutonium diversion.

The last step in the nuclear fuel cycle, waste management, remains one of the most controversial. The principal issue here is not so much the present danger as the danger to generations far in the future. Many nuclear wastes remain radioactive for thousands of years, beyond the span of any human institution. The technology for packaging the wastes so that they pose no current hazard is relatively straightforward. The difficulty lies both in being adequately confident that future generations are well protected and in making the political decision on how and where to proceed with waste storage. Permanent but potentially retrievable storage in deep stable geologic formations seems the best solution. In 1988 the U.S. government chose Yucca Mountain, a Nevada desert site with a thick section of porous volcanic rocks, as the nation's first permanent underground repository for more than 36,290 metric tons of nuclear waste. However, opposition from state residents and uncertainty that Yucca Mountain may not be completely insulated from earthquakes and other hazards has prolonged government studies. For example, a geological study by the U.S. Department of Energy detected water in several mineral samples taken at the Yucca Mountain site. The presence of water in these samples suggests that water may have once risen up through the mountain and later subsided. Because such an event could jeopardize the safety of a nuclear waste repository, the Department of Energy has funded more study of these fluid intrusions.

A $2 billion repository built in underground salt caverns near Carlsbad, New Mexico, is designed to store radioactive waste from the manufacture of nuclear weapons during the Cold War. This repository, located 655 meters (2,150 feet) underground, is designed to slowly collapse and encapsulate the plutonium-contaminated waste in the salt beds. Although the repository began receiving radioactive waste shipments in April 1999, environmentalists planned to file a lawsuit to close the Carlsbad repository.

The release of nuclear energy can occur at the low end of the binding energy curve (see accompanying chart) through the fusion of two light nuclei into a heavier one. The energy radiated by stars, including the Sun, arises from such fusion reactions deep in their interiors. At the enormous pressure and at temperatures above 15 million ° C (27 million ° F) existing there, hydrogen nuclei combine according to equation (1) and give rise to most of the energy released by the Sun.

Nuclear fusion was first achieved on earth in the early 1930s by bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with high-energy deuterons in a cyclotron (see Particle Accelerators). To accelerate the deuteron beam a great deal of energy is required, most of which appeared as heat in the target. As a result, no net useful energy was produced. In the 1950s the first large-scale but uncontrolled release of fusion energy was demonstrated in the tests of thermonuclear weapons by the United States, the USSR, the United Kingdom, and France. This was such a brief and uncontrolled release that it could not be used for the production of electric power.

In the fission reactions discussed earlier, the neutron, which has no electric charge, can easily approach and react with a fissionable nucleus-for example, uranium-235. In the typical fusion reaction, however, the reacting nuclei both have a positive electric charge, and the natural repulsion between them, called Coulomb repulsion, must be overcome before they can join. This occurs when the temperature of the reacting gas is sufficiently high-50 to 100 million ° C (90 to 180 million ° F). In a gas of the heavy hydrogen isotopes deuterium and tritium at such temperature, the fusion reaction

occurs, releasing about 17.6 MeV per fusion event. The energy appears first as kinetic energy of the helium-4 nucleus and the neutron, but is soon transformed into heat in the gas and surrounding materials.

If the density of the gas is sufficient-and at these temperatures the density need be only 10-5 atm, or almost a vacuum-the energetic helium-4 nucleus can transfer its energy to the surrounding hydrogen gas, thereby maintaining the high temperature and allowing subsequent fusion reactions, or a fusion chain reaction, to take place. Under these conditions, "nuclear ignition" is said to have occurred.

The basic problems in attaining useful nuclear fusion conditions are (1) to heat the gas to these very high temperatures and (2) to confine a sufficient quantity of the reacting nuclei for a long enough time to permit the release of more energy than is needed to heat and confine the gas. A subsequent major problem is the capture of this energy and its conversion to electricity.

At temperatures of even 100,000° C (180,000° F), all the hydrogen atoms are fully ionized. The gas consists of an electrically neutral assemblage of positively charged nuclei and negatively charged free electrons. This state of matter is called a plasma.

A plasma hot enough for fusion cannot be contained by ordinary materials. The plasma would cool very rapidly, and the vessel walls would be destroyed by the extreme heat. However, since the plasma consists of charged nuclei and electrons, which move in tight spirals around the lines of force of strong magnetic fields, the plasma can be contained in a properly shaped magnetic field region without reacting with material walls.

In any useful fusion device, the energy output must exceed the energy required to confine and heat the plasma. This condition can be met when the product of confinement time t and plasma density n exceeds about 1014. The relationship tn≥ 1014 is called the Lawson criterion.

Numerous schemes for the magnetic confinement of plasma have been tried since 1950 in the United States, Russia, the United Kingdom, Japan, and elsewhere. Thermonuclear reactions have been observed, but the Lawson number rarely exceeded 1012. One device, however-the tokamak, originally suggested in the USSR by Igor Tamm and Andrey Sakharov-began to give encouraging results in the early 1960s.

The confinement chamber of a tokamak has the shape of a torus, with a minor diameter of about 1 m (about 3.3 ft) and a major diameter of about 3 m (about 9.8 ft). A toroidal (donut-shaped) magnetic field of about 50,000 gauss is established inside this chamber by large electromagnets. A longitudinal current of several million amperes is induced in the plasma by the transformer coils that link the torus. The resulting magnetic field lines, spirals in the torus, stably confine the plasma.

Based on the successful operation of small tokamaks at several laboratories, two large devices were built in the early 1980s, one at Princeton University in the United States and one in the USSR. The enormous magnetic fields in a tokamak subject the plasma to extremely high temperatures and pressures, forcing the atomic nuclei to fuse. As the atomic nuclei are fused together, an extraordinary amount of energy is released. During this fusion process, the temperature in the tokamak reaches three times that of the Sun’s core.

Another possible route to fusion energy is that of inertial confinement. In this concept, the fuel-tritium or deuterium-is contained within a tiny glass sphere that is then bombarded on several sides by a pulsed laser or heavy ion beam. This causes an implosion of the glass sphere, setting off a thermonuclear reaction that ignites the fuel. Several laboratories in the United States and elsewhere are currently pursuing this possibility. In the late 1990s, many researchers concentrated on the use of beams of heavy ions, such as barium ions, rather than lasers to trigger inertial-confinement fusion. Researchers chose heavy ion beams because heavy ion accelerators can produce intense ion pulses at high repetition rates and because heavy ion accelerators are extremely efficient at converting electric power into ion beam energy, thus reducing the amount of input power. Also in comparison to laser beams, ion beams can penetrate the glass sphere and fuel more effectively to heat the fuel.

Progress in fusion research has been promising, but the development of practical systems for creating a stable fusion reaction that produces more power than it consumes will probably take decades to realize. The research is expensive, as well. However, some progress was made in the early 1990s. In 1991, for the first time ever, a significant amount of energy-about 1.7 million watts-was produced from controlled nuclear fusion at the Joint European Torus (JET) Laboratory in England. In December 1993, researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 million watts of power. However, both the JET and the Tokamak Fusion Test Reactor consumed more energy than they produced during their operation.

If fusion energy does become practical, it offers the following advantages: (1) a limitless source of fuel, deuterium from the ocean; (2) no possibility of a reactor accident, as the amount of fuel in the system is very small; and (3) waste products much less radioactive and simpler to handle than those from fission systems.

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Nuclear Weapons

Nuclear Weapons, explosive devices designed to release nuclear energy on a large scale, used primarily in military applications. The first atomic bomb (or A-bomb), which was tested on July 16, 1945, at Alamogordo, New Mexico, represented a completely new type of explosive. All explosives prior to that time derived their power from the rapid burning or decomposition of some chemical compound. Such chemical processes release only the energy of the outermost electrons in the atom. See Atom.

Nuclear explosives, on the other hand, involve energy sources within the core, or nucleus, of the atom. The A-bomb gained its power from the splitting, or fission, of all the atomic nuclei in several kilograms of plutonium. A sphere about the size of a baseball produced an explosion equal to 20,000 tons of TNT.

The A-bomb was developed, constructed, and tested by the Manhattan Project, a massive United States enterprise that was established in August 1942, during World War II. Many prominent American scientists, including the physicists Enrico Fermi and J. Robert Oppenheimer and the chemist Harold Urey, were associated with the project, which was headed by a U.S. Army engineer, then-Brigadier General Leslie R. Groves.

After the war, the U.S. Atomic Energy Commission became responsible for the oversight of all nuclear matters, including research on hydrogen bombs. In these bombs the source of energy is the fusion process, in which nuclei of the isotopes of hydrogen combine to form a heavier helium nucleus (see Thermonuclear, or Fusion, Weapons below). This weapons research resulted in the production of bombs that range in power from a fraction of a kiloton (1,000 tons of TNT equivalent) to many megatons (1 megaton equals 1 million tons of TNT equivalent). Furthermore, the physical size of a nuclear bomb was drastically reduced, permitting the development of nuclear artillery shells and small missiles that can be fired from portable launchers in the field. Although nuclear bombs were originally developed as strategic weapons to be carried by large bombers, nuclear weapons are now available for a variety of both strategic and tactical applications. Not only can they be delivered by different types of aircraft, but rockets and guided missiles of many sizes can now carry nuclear warheads and can be launched from the ground, the air, or underwater. Large rockets can carry multiple warheads for delivery to separate targets. See also ICBM; SLBM; MIRV.

In 1905 Albert Einstein published his special theory of relativity. According to this theory, the relation between mass and energy is expressed by the equation E = mc2, which states that a given mass (m) is associated with an amount of energy (E) equal to this mass multiplied by the square of the speed of light (c). A very small amount of matter is equivalent to a vast amount of energy. For example, 1 kg (2.2 lb) of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNT.

In 1938 German chemists Otto Hahn and Fritz Strassmann split the uranium atom into two roughly equal parts by bombardment with neutrons. As a result of these experiments, the Austrian physicist Lise Meitner, with her nephew, the British physicist Otto Robert Frisch, went on to explain the process of nuclear fission in 1939, placing the release of atomic energy within reach.

When a uranium or other suitable nucleus fissions, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes it possible to achieve a self-sustaining series of nuclear fissions; the neutrons that are emitted in fission produce a chain reaction, with continuous release of energy.

The light isotope of uranium, uranium-235, is easily split by the fission neutrons and, upon fission, emits an average of about 2.5 neutrons. One neutron per generation of nuclear fissions is necessary to sustain the chain reactions. Others may be lost by escape from the mass of chain-reacting material, or they may be absorbed in impurities or in the heavy uranium isotope, uranium-238, if it is present. Any substance capable of sustaining a fission chain reaction is known as a fissile material.

A small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, would not sustain a chain reaction. Too many neutrons escape through the surface area, which is relatively large compared with its volume, and thus are lost to the chain reaction. In a mass of uranium-235 about the size of a baseball, however, the number of neutrons lost through the surface is compensated for by the neutrons generated in additional fissions taking place within the sphere. The minimum amount of fissile material (of a given shape) required to maintain the chain reaction is known as the critical mass. Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fissions increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy. In an atomic bomb, therefore, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape.

If every atom in 0.5 kg (1.1 lb) of uranium were to split, the energy produced would equal the explosive power of 9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process would be 100 percent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical assembly.

Various systems have been devised to detonate the atomic bomb. The simplest system is the gun-type weapon, in which a projectile made of fissile material is fired at a target of the same material so that the two weld together into a supercritical assembly. The atomic bomb exploded by the United States over Hiroshima, Japan, on August 6, 1945, was a gun-type weapon. It had the energy of anywhere between 12.5 and 15 kilotons of TNT. Three days later the United States dropped a second atomic bomb over Nagasaki, Japan, with the energy equivalent of about 20 kilotons of TNT.

A more complex method, known as implosion, is used in a spherically shaped weapon. The outer part of the sphere consists of a layer of closely fitted and specially shaped devices, called lenses, consisting of high explosive and designed to concentrate the blast toward the center of the bomb. Each segment of the high explosive is equipped with a detonator, which in turn is wired to all other segments. An electrical impulse explodes all the chunks of high explosive simultaneously, resulting in a detonation wave that converges toward the core of the weapon. At the core is a sphere of fissile material, which is compressed by the powerful, inwardly directed pressure, or implosion. The density of the metal is increased, and a supercritical assembly is produced. The Alamogordo test bomb, as well as the one dropped by the United States on Nagasaki, Japan, on August 9, 1945, were of the implosion type. Each was equivalent to about 20 kilotons of TNT.

Regardless of the method used to attain a supercritical assembly, the chain reaction proceeds for about a millionth of a second, liberating vast amounts of heat energy. The extremely fast release of a very large amount of energy in a relatively small volume causes the temperature to rise to tens of millions of degrees. The resulting rapid expansion and vaporization of the bomb material causes a powerful explosion.

Much experimentation was necessary to make the production of fissile material practical.

The fissile isotope uranium-235 accounts for only 0.7 percent of natural uranium; the remainder is composed of the heavier uranium-238. No chemical methods suffice to separate uranium-235 from ordinary uranium, because both uranium isotopes are chemically identical. A number of techniques were devised to separate the two, all of which depend in principle on the slight difference in weight between the two types of uranium atoms.

A huge gaseous-diffusion plant was built during World War II in Oak Ridge, Tennessee. This plant was enlarged after the war, and two similar plants were built near Paducah, Kentucky, and Portsmouth, Ohio. The feed material for this type of plant consists of extremely corrosive uranium hexafluoride gas. The gas is pumped against barriers that have many millions of tiny holes, through which the lighter molecules, which contain uranium-235 atoms, diffuse at a slightly greater rate than the heavier molecules, containing uranium-238 (see Diffusion). After the gas has been cycled through thousands of barriers, known as stages, it is highly enriched in the lighter isotope of uranium. The final product is weapon-grade uranium, containing more than 90 percent uranium-235.

Although the heavy uranium isotope uranium-238 will not sustain a chain reaction, it can be converted into a fissile material by bombarding it with neutrons and transforming it into a new species of element. When the uranium-238 atom captures a neutron in its nucleus, it is transformed into the heavier isotope uranium-239. This nuclear species quickly disintegrates to form neptunium-239, an isotope of element 93 (see Neptunium). Another disintegration transmutes this isotope into an isotope of element 94, called plutonium-239. Plutonium-239, like uranium-235, undergoes fission after the absorption of a neutron and can be used as a bomb material. Producing plutonium-239 in large quantities requires an intense source of neutrons; the source is provided by the controlled chain reaction in a nuclear reactor. See Nuclear Chemistry.

During World War II nuclear reactors were designed to provide neutrons to produce plutonium. Reactors capable of manufacturing large quantities of plutonium were established in Hanford, Washington, and near Aiken, South Carolina.

Even before the first atomic bomb was developed, scientists realized that a type of nuclear reaction different from the fission process was theoretically possible as a source of nuclear energy. Instead of using the energy released as a result of a chain reaction in fissile material, nuclear weapons could use the energy liberated in the fusion of light elements. This process is the opposite of fission, since it involves the fusing together of the nuclei of isotopes of light atoms such as hydrogen. It is for this reason that the weapons based on nuclear-fusion reactions are often called hydrogen bombs, or H-bombs. Of the three isotopes of hydrogen the two heaviest species, deuterium and tritium, combine most readily to form helium. Although the energy release in the fusion process is less per nuclear reaction than in fission, 0.5 kg (1.1 lb) of the lighter material contains many more atoms; thus, the energy liberated from 0.5 kg (1.1 lb) of hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or almost three times as much as from uranium. This estimate, however, is based on complete fusion of all hydrogen atoms. Fusion reactions occur only at temperatures of several millions of degrees, the rate increasing enormously with increasing temperature; such reactions consequently are known as thermonuclear (heat-induced) reactions. Strictly speaking, the term thermonuclear implies that the nuclei have a range (or distribution) of energies characteristic of the temperature. This plays an important role in making rapid fusion reactions possible by an increase in temperature.

Development of the hydrogen bomb was impossible before the perfection of A-bombs, for only the latter could yield that tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic scientists regarded the A-bomb as the trigger of the projected thermonuclear device.

Following developmental tests in the spring of 1951 at the U.S. Enewetak Proving Grounds in the Marshall Islands during Operation Greenhouse, a full-scale, successful experiment was conducted on November 1, 1952, with a fusion-type device. This test, called Mike, which was part of Operation Ivy, produced an explosion with power equivalent to several million tons of TNT (that is, several megatons). The Soviet Union detonated a thermonuclear weapon in the megaton range in August 1953. On March 1, 1954, the United States exploded a fusion bomb with a power of 15 megatons. It created a glowing fireball, more than 4.8 km (more than 3 mi) in diameter, and a huge mushroom cloud, which quickly rose into the stratosphere.

The March 1954 explosion led to worldwide recognition of the nature of radioactive fallout. The fallout of radioactive debris from the huge bomb cloud also revealed much about the nature of the thermonuclear bomb. Had the bomb been a weapon consisting of an A-bomb trigger and a core of hydrogen isotopes, the only persistent radioactivity from the explosion would have been the result of the fission debris from the trigger and from the radioactivity induced by neutrons in coral and seawater. Some of the radioactive debris, however, fell on the Lucky Dragon, a Japanese vessel engaged in tuna fishing about 160 km (about 100 mi) from the test site. This radioactive dust was later analyzed by Japanese scientists. The results demonstrated that the bomb that dusted the Lucky Dragon with fallout was more than just an H-bomb.

The thermonuclear bomb exploded in 1954 was a three-stage weapon. The first stage consisted of a big A-bomb, which acted as a trigger. The second stage was the H-bomb phase resulting from the fusion of deuterium and tritium within the bomb. In the process helium and high-energy neutrons were formed. The third stage resulted from the impact of these high-speed neutrons on the outer jacket of the bomb, which consisted of natural uranium, or uranium-238. No chain reaction was produced, but the fusion neutrons had sufficient energy to cause fission of the uranium nuclei and thus added to the explosive yield and also to the radioactivity of the bomb residues.

The effects of nuclear weapons were carefully observed, both after the bombings of Hiroshima and Nagasaki and after many test explosions in the 1950s and early 1960s.

As is the case with explosions caused by conventional weapons, most of the damage to buildings and other structures from a nuclear explosion results, directly or indirectly, from the effects of blast. The very rapid expansion of the bomb materials produces a high-pressure pulse, or shock wave, that moves rapidly outward from the exploding bomb. In air, this shock wave is called a blast wave because it is equivalent to and is accompanied by powerful winds of much greater than hurricane force. Damage is caused both by the high excess (or overpressure) of air at the front of the blast wave and by the extremely strong winds that persist after the wave front has passed. The degree of blast damage suffered on the ground depends on the TNT equivalent of the explosion; the altitude at which the bomb is exploded, referred to as the height of burst; and the distance of the structure from ground zero, that is, the point directly under the bomb. For the 20-kiloton A-bombs detonated over Japan, the height of burst was about 580 m ( about 1,900 ft), because it was estimated that this height would produce a maximum area of damage. If the TNT equivalent had been larger, a greater height of burst would have been chosen.

Assuming a height of burst that will maximize the damage area, a 10-kiloton bomb will cause severe damage to wood-frame houses, such as are common in the United States, to a distance of more than 1.6 km (more than 1 mi) from ground zero and moderate damage as far as 2.4 km (1.5 mi). (A severely damaged house probably would be beyond repair.) The damage radius increases with the power of the bomb, approximately in proportion to its cube root. If exploded at the optimum height, therefore, a 10-megaton weapon, which is 1,000 times as powerful as a 10-kiloton weapon, will increase the distance tenfold, that is, out to 17.7 km (11 mi) for severe damage and 24 km (15 mi) for moderate damage of a frame house.

The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot incandescent mass of gas called a fireball. For a 10-kiloton explosion in the air, the fireball will attain a maximum diameter of about 300 m (about 1,000 ft); for a 10-megaton weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (or heat) radiation is emitted from the fireball and spreads out over a large area, but with steadily decreasing intensity. The amount of heat energy received a certain distance from the nuclear explosion depends on the power of the weapon and the state of the atmosphere. If the visibility is poor or the explosion takes place above clouds, the effectiveness of the heat flash is decreased. The thermal radiation falling on exposed skin can cause what are called flash burns. A 10-kiloton explosion in the air can produce moderate (second-degree) flash burns, which require some medical attention, as far as 2.4 km (1.5 mi) from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km (more than 20 mi). Milder burns of bare skin would be experienced even farther out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque object. Flash burns occur only when the bare skin is directly exposed, or if the clothing is too thin to absorb the thermal radiation.

The heat radiation can initiate fires in dry, flammable materials, for example, paper and some fabrics, and such fires may spread if conditions are suitable. The evidence from the A-bomb explosions over Japan indicates that many fires, especially in the area near ground zero, originated from secondary causes, such as electrical short circuits, broken gas lines, and upset furnaces and boilers in industrial plants. The blast damage produced debris that helped to maintain the fires and denied access to fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary effect of the blast wave.

Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a fire storm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraft, which produces strong winds drawn in toward the center of the burning area. These winds fan the flame and convert the area into a holocaust in which everything flammable is destroyed. Inasmuch as the flames are drawn inward, however, the area over which such a fire spreads may be limited.

Besides heat and blast, an exploding nuclear bomb has a unique effect-it releases penetrating nuclear radiation, which is quite different from thermal (or heat) radiation (see Radioactivity). When absorbed by the body, nuclear radiation can cause serious injury. For an explosion high in the air, the injury range for these radiations is less than for blast and fire damage or flash burns. In Japan, however, many individuals who were protected from blast and burns succumbed later to radiation injury.

Nuclear radiation from an explosion may be divided into two categories, namely, prompt radiation and residual radiation. The prompt radiation consists of an instantaneous burst of neutrons and gamma rays, which travel over an area of several square miles. Gamma rays are identical in effect to X rays (see X Ray). Both neutrons and gamma rays have the ability to penetrate solid matter, so that substantial thicknesses of shielding materials are required.

The residual nuclear radiation, generally known as fallout, can be a hazard over very large areas that are completely free from other effects of a nuclear explosion. In bombs that gain their energy from fission of uranium-235 or plutonium-239, two radioactive nuclei are produced for every fissile nucleus split. These fission products account for the persistent radioactivity in bomb debris, because many of the atoms have half-lives measured in days, months, or years.

Two distinct categories of fallout, namely, early and delayed, are known. If a nuclear explosion occurs near the surface, earth or water is taken up into a mushroom-shaped cloud and becomes contaminated with the radioactive weapon residues. The contaminated material begins to descend within a few minutes and may continue to fall for about 24 hours, covering an area of thousands of square miles downwind from the explosion. This constitutes the early fallout, which is an immediate hazard to human beings. No early fallout is associated with high-altitude explosions. If a nuclear bomb is exploded well above the ground, the radioactive residues rise to a great height in the mushroom cloud and descend gradually over a large area.

Human experience with radioactive fallout has been minimal. The principal known case histories have been derived from the accidental exposure of fishermen and local residents to the fallout from the 15-megaton explosion that occurred on March 1, 1954. The nature of radioactivity, however, and the immense areas contaminable by a single bomb undoubtedly make radioactive fallout potentially one of the most lethal effects of nuclear weapons.

Besides the blast and radiation damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a catastrophic global effect on climate. This possibility, proposed in a paper published by an international group of scientists in December 1983, has come to be known as the "nuclear winter" theory. According to these scientists, the explosion of not even one-half of the combined number of warheads in the United States and Russia would throw enormous quantities of dust and smoke into the atmosphere. The amount could be sufficient to block off sunlight for several months, particularly in the northern hemisphere, destroying plant life and creating a subfreezing climate until the dust dispersed. The ozone layer might also be affected, permitting further damage as a result of the sun’s ultraviolet radiation. Were the results sufficiently prolonged, they could spell the virtual end of human civilization. The nuclear winter theory has since become the subject of enormous controversy. It found support in a study released in December 1984 by the U.S. National Research Council, and other groups have undertaken similar research. In 1985, however, the U.S. Department of Defense released a report acknowledging the validity of the concept but saying that it would not affect defense policies.

On the average, about 50 percent of the power of an H-bomb results from thermonuclear-fusion reactions and the other 50 percent from fission that occurs in the A-bomb trigger and in the uranium jacket. A clean H-bomb is defined as one in which a significantly smaller proportion than 50 percent of the energy arises from fission. Because fusion does not produce any radioactive products directly, the fallout from a clean weapon is less than that from a normal or average H-bomb of the same total power. If an H-bomb were made with no uranium jacket but with a fission trigger, it would be relatively clean. Perhaps as little as 5 percent of the total explosive force might result from fission; the weapon would thus be 95 percent clean. The enhanced-radiation fusion bomb, also called the neutron bomb, which has been tested by the United States and other nuclear powers, does not release long-lasting radioactive fission products. However, the large number of neutrons released in thermonuclear reactions is known to induce radioactivity in materials, especially earth and water, within a relatively small area around the explosion. Thus the neutron bomb is considered a tactical weapon because it can do serious damage on the battlefield, penetrating tanks and other armored vehicles and causing death or serious injury to exposed individuals, without producing the radioactive fallout that endangers people or structures miles away. See also Arms Control, International; Guided Missiles; Warfare.

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