The atom, considered the basic unit of matter, is composed of a nucleus and one or more electrons. Shown below is a crude representation of the atom. The structure of an atom may be comprised of three particles: the negatively charged electron, the positively charged proton, and the uncharged neutron. The protons and neutrons in an atom make up a small, very dense, positively charged core called the nucleus, with dimensions of the order of 10−15 m. The positive charge on the nucleus is a result of the presence of protons in the nucleus, as represented by the red circles with plus signs. Protons are particles with a unit positive charge. Neutrons, as indicated by the name are neutral, that is they have no charge, and are represented by the blue circles. The nucleus is at the center of the atom. Surrounding the nucleus are rapidly moving, negatively charged particles called electrons, represented by the yellow circles with negative signs. The electrons that orbit the nucleus can be thought of as an electron cloud. An electron is much smaller than protons and neutrons, and have a unit negative charge. Electrons extend out to distances of the order of 10−10 m from the nucleus. If an atom were a few kilometers across, its nucleus would be the size of a baseball.

Atoms are minuscule objects with proportionately minuscule masses. Over 99.9% of an atom's mass is concentrated in the nucleus. The masses of the proton and neutron are nearly equal and are roughly 2000 times the mass of the electron. (click here for proton, neutron, and electron masses). Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons that are bound to atoms possess a set of stable energy levels, or orbitals.

An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element. The negative charge of the electron has (within experimental error) exactly the same magnitude as the positive charge of the proton. In a neutral atom the number of electrons equals the number of protons in the nucleus, and the net electric charge (the algebraic sum of all charges) is exactly zero. If one or more electrons are removed, the remaining positively charged structure is called a positive ion. A negative ion is an atom that has gained one or more electrons. This gaining or losing of electrons is called ionization.

All atoms of an element have the same number of electrons around the nucleus; atoms of different elements have different numbers of electrons. The atomic number of an element distinguishes atoms of one element from atoms of another element and indicates which element an atom is. An element is a substance composed of atoms that all have the same atomic number (same number of protons). The negatively charged electrons are held within the atom by the attractive electric forces exerted on them by the positively charged nucleus. The protons and neutrons are held within the stable atomic nuclei by an attractive interaction, called the nuclear force, that overcomes the electric repulsion of the protons. The nuclear force has a short range, of the order of nuclear dimensions, and its effects do not extend far beyond the nucleus. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron).

An atom is one of the basic units of matter. Everything around us is made up of atoms. An atom is incredibly tiny--more than a million times smaller than the thickness of a human hair. The smallest speck that can be seen under an ordinary microscope contains more than 10 billion atoms. The diameter of an atom ranges from about 0.1 to 0.5 nanometer.

Atoms form the building blocks of the simplest substances, the chemical elements. Familiar elements include hydrogen, oxygen, iron, and lead. Each element consists of one basic kind of atom. Compounds are more complex substances made of two or more kinds of atoms linked in units called molecules. Water, for example, is a compound in which each molecule consists of two atoms of hydrogen linked to one atom of oxygen.

Atoms vary greatly in weight, but they are all about the same size. For example, an atom of plutonium, the heaviest element found in nature, weighs more than 200 times as much as an atom of hydrogen, the lightest known element. However, the diameter of a plutonium atom is only about 3 times that of a hydrogen atom.

The Parts of an Atom

Tiny as atoms are, they consist of even more minute particles. The three basic types are protons, neutrons, and electrons. Each atom has a definite number of these subatomic particles. The protons and neutrons are crowded into the nucleus, an exceedingly tiny region at the center of the atom. If a hydrogen atom were about 4 miles (6.4 kilometers) in diameter, its nucleus would be no bigger than a tennis ball. The rest of an atom outside the nucleus is mostly empty space. The electrons whirl through this space, completing billions of trips around the nucleus each millionth of a second. The fantastic speed of the electrons makes atoms behave as if they were solid, much as the fast-moving blades of a fan prevent a pencil from being pushed through them.

Atoms are often compared to the solar system, with the nucleus corresponding to the sun and the electrons corresponding to the planets that orbit the sun. This comparison is not completely accurate, however. Unlike the planets, the electrons do not follow regular, orderly paths. In addition, the protons and neutrons constantly move about at random inside the nucleus.

The Nucleus makes up nearly all the mass of an atom. Mass is the quantity of matter in an atom. Each proton has a mass roughly equal to that of 1,836 electrons. It would take 1,839 electrons to equal a neutron's mass. Each proton carries one unit of positive electric charge. Each electron carries one unit of negative charge. Neutrons have no charge. Under most conditions, an atom has the same number of protons and electrons, and so the atom is electrically neutral.

Protons and neutrons are about 100,000 times smaller than atoms, but they are in turn made up of even smaller particles called quarks. Each proton and neutron consists of three quarks. In the laboratory, scientists can cause quarks to combine and form other kinds of subatomic particles besides protons and neutrons. All these other particles break down and change into ordinary particles in a small fraction of a second. Thus, none of them is found in ordinary atoms. However, scientists first learned that protons and neutrons consist of quarks through the study of other subatomic particles.

The Electrons, unlike the protons and neutrons, do not seem to have smaller parts. Electrons have very little mass. The mass of an electron in grams may be written with a decimal point followed by 27 zeros and a 9.

Opposite electric charges attract. The positively charged nucleus therefore exerts a force on the negatively charged electrons that keeps them within the atom. However, each electron has energy and so is able to resist the attraction of the nucleus. The more energy an electron has, the farther from the nucleus it will be. Thus, electrons are arranged in shells at various distances from the nucleus according to how much energy they have. Electrons with the least energy are in inner shells, and those with more energy are in outer shells.

Each electron shell is labeled with a number. The shell closest to the nucleus is called shell 1. The other shells, in order of increasing distance from the nucleus, are shells 2, 3, 4, 5, 6, and 7. The shells are sometimes named by the letters K, L, M, N, O, P, and Q. Each shell can hold only a limited number of electrons. Shell 1 can hold no more than 2 electrons. Shell 2 can hold 8 electrons, shell 3 can hold 18, and shell 4 can hold 32. In theory, shell 5 can hold 50 electrons, shell 6 can hold 72, and shell 7 can hold 98. However, these outer shells are never completely filled.

The Properties of Atoms

The Atomic Number tells how many protons an atom has. All atoms of the same element have the same number of protons. For example, every hydrogen atom has a single proton, and so the atomic number of hydrogen is 1. The atomic numbers for other natural elements range successively up to 92 for uranium, which has 92 protons in each atom. Tiny amounts of plutonium, which has an atomic number of 94, also occur naturally. Elements whose atoms have more than 92 protons can be created in the laboratory.

The atomic number determines an element's place in the periodic table. This table organizes the elements into groups with similar chemical properties.

The Mass Number is the sum of the protons and neutrons in an atom. Although all atoms of an element have the same number of protons, they may have different numbers of neutrons. Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

Most of the elements in nature have more than one isotope. Hydrogen, for example, has three. In the most common hydrogen isotope, the nucleus consists only of a proton. In the two other hydrogen isotopes, the nucleus consists of one or two neutrons in addition to the proton. Scientists use the mass number to distinguish the three isotopes as hydrogen 1, hydrogen 2, and hydrogen 3. They also refer to hydrogen 1 as protium, to hydrogen 2 as deuterium, and to hydrogen 3 as tritium.

In most lighter elements, the nucleus of each atom contains about an equal number of protons and neutrons. Most heavier elements, however, have more neutrons than protons. The heaviest elements have about 3 neutrons for every 2 protons. For example, uranium 238 has 146 neutrons and 92 protons.

Atoms that have the same mass number but different atomic numbers are called isobars. Thus, isobars are atoms of different elements. For example, the isobars argon and calcium have a mass number of 40, but argon's atomic number is 18 and calcium's is 20.

Atomic Weight is the weight of an atom expressed in atomic mass units (amu). One amu, also called a dalton, equals 1/12 the weight of an atom of carbon 12. For most atoms, the weight in amu is extremely close to the mass number. Atomic mass units are very small. There are 602 billion trillion amu in a gram.

Scientists determine the atomic weight of an element with more than one isotope by averaging the weights of all the isotopes in the proportions in which they occur in nature. For example, the atomic weight of chlorine is 35.453 amu. This value is an average for the two isotopes chlorine 35 (atomic weight 34.96885) and chlorine 37 (atomic weight 36.96590) in their natural proportions.

Although an atom is normally electrically neutral, it can lose or gain a few electrons in some chemical reactions or in a collision with an electron or another atom. This gain or loss of electrons produces an electrically charged atom called an ion. An atom that loses electrons becomes a positive ion, and an atom that gains electrons becomes a negative ion. The gain or loss of electrons is called ionization.

The chemical behavior of an atom is determined largely by the number of electrons in its outermost shell. When atoms combine and form molecules, electrons in the outermost shell are either transferred from one atom to another or shared between atoms. The number of electrons involved in this process is called the valence. The atoms of some elements can have more than one valence, depending on the number and kind of atoms with which they combine.

If an atom tends to lose electrons to other atoms, its valence is positive. If an atom tends to gain electrons, its valence is negative. For example, sodium tends to lose its one electron and thus has a valence of +1. Chlorine tends to accept one electron from another atom and so has a valence of -1. A molecule of ordinary table salt consists of one atom of sodium linked to one atom of chlorine. The sodium atom donates the electron that chlorine is able to accept.

In some atoms, the nucleus can change naturally. Such an atom is called radioactive. The change in the nucleus may be only in the arrangement of the protons and neutrons. Or the actual number of protons and neutrons may change. When a nucleus changes, it gives off radiation. This radiation consists of alpha or beta particles or gamma rays. Atoms of uranium, radium, and all other elements heavier than bismuth are radioactive. Some isotopes of lighter elements are also radioactive. In addition, physicists can create radioactive isotopes of nearly all elements in a laboratory by bombarding atoms with subatomic particles.

The type of radiation given off by a radioactive nucleus depends on the way the nucleus changes. Gamma rays are given off if only the arrangement of the protons and neutrons in the nucleus changes. But alpha or beta radiation is given off if the number of protons and neutrons in the nucleus changes. The atom then becomes an atom of a different element. This process is called transmutation or radioactive decay.

The Forces Within an Atom

The field of physics called quantum mechanics deals with the forces inside an atom and the motions of subatomic particles. This field began in 1913, when the Danish physicist Niels Bohr used the quantum theory to explain the motion of electrons in atoms.

Electron Energy Levels. According to quantum mechanics, electrons cannot have just any amount of energy. Instead, electrons are restricted to a limited set of motions, each of which has a specific value of energy. These motions are called quantum states or energy levels. When an electron is in a given quantum state, it does not absorb or give off energy. For this reason, an atom can gain or lose energy only if one or more electrons change their quantum state.

Just as water always seeks its lowest possible level, electrons seek the state of lowest energy. However, only one electron at a time can occupy each quantum state. If the lower states are filled, other electrons are forced to occupy higher states. If all electrons are in the lowest possible state, the atom is in its ground state. This condition is normal for atoms at ordinary temperatures.

When matter is heated to temperatures higher than a few hundred degrees, energy is available to raise one or more electrons to a higher energy level. The atom is then in an excited state. However, atoms rarely remain in an excited state for more than a fraction of a second. An excited electron almost immediately drops to a lower state and continues dropping until the atom returns to its ground state. At each succeeding drop, the electron gives off a tiny packet of radiant energy called a photon. The energy of the photon equals the difference between the two energy levels of the electron. The photons given off by electrons are detected as visible light and other forms of electromagnetic radiation.

Bohr originally described the quantum states of electrons as orbits like those of the planets around the sun. However, physicists now know that this description is incorrect because an electron is not simply a particle. An electron also has some characteristics of a wave. It is difficult to imagine how something could be both a particle and a wave. This difficulty is one of the problems scientists have in trying to describe the atom to nonscientists. To do so, scientists must use familiar ideas based on our knowledge of the world as we observe it. But conditions inside the tiny atom differ greatly from those in our everyday world. For this reason, physicists can describe the motions of electrons accurately and completely only in mathematical terms.

The quantum rules that govern the motions of electrons also apply to the motions of protons and neutrons inside the nucleus. However, the force that keeps the nuclear particles together differs greatly from the electrical attraction that holds the electrons within the atom. Each nuclear particle is attracted to its nearest neighbor by what is called the nuclear force or, sometimes, the strong interaction. Like electric charges repel each other. However, the powerful nuclear force overcomes the mutual repulsion of the positively charged protons. It thus keeps the nucleus from flying apart. This force dies off quickly, however, unless the nuclear particles are extremely close together. Electrons are immune to the nuclear force.

The nuclear force is highly complicated, and no exact mathematical description of it has been formulated. Nevertheless, a theory known as the nuclear shell model provides reasonably accurate estimates of the energy levels in the nucleus.

One neutron and one proton can occupy each quantum state in the nucleus. For this reason, a light nucleus has a nearly equal number of protons and neutrons. But a proton and a neutron in the same state do not have the same amount of energy. Each proton is electrically repelled by all other protons in the nucleus, which increases its energy. In a nucleus with many protons, the difference in energy levels between protons and neutrons is considerable, and more low-energy states are available for neutrons than for protons. This is why a heavy nucleus has more neutrons than protons.

How Scientists Study Atoms

Scientists use a variety of instruments and techniques to study atoms. The devices and methods used depend on whether the researchers are studying atoms themselves, electrons, nuclear particles, or quarks.

Researchers use X rays to study the arrangements of atoms in regular, repeated patterns, such as in crystals. When X rays pass through a crystal, the atoms in the crystal diffract (spread out) the X rays in a certain way. These diffracted rays produce patterns on photographic film that reveal how far apart the atoms are and how they are arranged. Extremely powerful scanning electron microscopes, scanning tunneling microscopes, and field-emission microscopes enable scientists to observe the positions of individual atoms. However, these instruments cannot reveal any details of the structure of atoms.

Scientists study the energies of electrons chiefly by analyzing the light given off by atoms in heated gases. Instruments called spectrometers break up the light into a spectrum with a separate line for each wavelength of light. Each wavelength is related to the difference in energy between two quantum states in the atom. After determining the wavelengths, scientists can draw up a complete list of energy levels. With the aid of quantum mechanics, they can then obtain a description of the electron motions in the atom.

Most of what scientists know about nuclear structure has come from experiments with particle accelerators. These devices bombard the nucleus with beams of high-energy electrons or protons. The swift-moving electrons or protons can disrupt the motion of particles in the nucleus and occasionally even knock some of them loose. In some experiments, whole nuclei are accelerated and smashed into stationary nuclei. Nuclear physicists have developed a wide variety of detectors for observing the particles that emerge from these collisions. Most of the detectors produce an electric signal when a particle passes through them.

Particle accelerators are also used to study the behavior of quarks. But such studies require particles with much greater energies than those used to study atomic nuclei. Thus, much more powerful accelerators are required.

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