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Tom’s Infinite Science Archive: Atom and Atomic Theory


Atom and Atomic Theory, the study of the nature of atoms and the forces which hold them together. In ancient Greek philosophy the word atom was used to describe the smallest bit of matter that could be conceived. This "fundamental particle," to use the present-day term for this concept, was thought of as indestructible; in fact, the Greek word for atom means "not divisible." Knowledge about the size and nature of the atom grew slowly throughout the centuries when people were content merely to speculate about it.

Avogadro's Law

The study of gases attracted the attention of the Italian physicist Amedeo Avogadro, who in 1811 formulated an important law bearing his name. This law states that equal volumes of different gases contain the same number of molecules when compared under the same conditions of temperature and pressure. Given these conditions, two identical bottles, one filled with oxygen and the other with helium, will contain exactly the same number of molecules. Twice as many atoms of oxygen will be present, however, because oxygen is diatomic.

Atomic Weight

Measurement of the weights of standard volumes (that is, the densities) of different gases permits direct comparison of the weights of individual gas molecules. When oxygen is taken as a standard and the oxygen atom is assigned a value of 16.0000 atomic mass units, helium is found to have an atomic weight of 4.003 atomic mass units, fluorine 19.000, and sodium 22.997. (Note that it is customary to speak of "atomic weights," although "atomic masses" would perhaps be more accurate. Mass is a measure of the quantity of matter in a body, whereas weight is the force exerted on the body by the influence of gravity. Thus, "atomic weight" is measured in atomic mass units. In processes that occur within the nuclei of atoms, such as nuclear fission, mass is converted into energy.)

The observation that many atomic weights are close to whole numbers led the British chemist William Prout to suggest in 1816 that all elements might be composed of hydrogen atoms. Subsequent measurements of atomic weights revealed that chlorine, for example, has an atomic weight of 35.455. The discovery of such fractional atomic weights appeared to invalidate Prout's hypothesis until a century later, when it was discovered that the atoms of most elements do not all have the same weight. Atoms of the same element that differ in weight are known as isotopes. In the case of chlorine two isotopes occur in nature. Experiments show that chlorine is a mixture of three parts of chlorine-35 for every one part of the heavier chlorine-37 isotope. This proportion accounts for the observed atomic weight of chlorine. Atomic scientists can measure isotopes with great precision. For example, the light isotope of chlorine is measured at 34.97867 amu.

The standard used for the calculation of atomic weights has recently been changed. During the first part of the 20th century it was customary to use natural oxygen as the standard against which atomic weights or masses were computed; oxygen was assigned an integral atomic weight of 16. This standard was used by chemists even after the rare isotopes of oxygen (oxygen-17 and oxygen-18) were discovered in 1929, because the small amounts of these isotopes in natural oxygen are relatively, although not absolutely, in constant proportion to the abundant isotope, oxygen-16. Physicists found it easier, however, to compute atomic masses against only the oxygen-16 isotope. This method resulted in two slightly different tables of atomic weights or masses. The situation was resolved in the early 1960s, when the international unions of chemistry and physics agreed on a single new standard, the abundant isotope of carbon, carbon-12. The new standard completely replaced the two earlier standards for all scientists. The new standard is particularly appropriate because carbon-12 is often used as a reference standard in computations of atomic masses using the mass spectrometer. Moreover, the table of atomic weights based on carbon-12 is in close agreement with the old table based on natural oxygen.

Periodic Table

By the middle of the 19th century several chemists recognized that similarities in the chemical properties of various elements implied a regularity that might be illustrated by arranging the elements in a tabular or periodic form. The Russian chemist Dmitry Mendeleyev proposed a chart of elements called the periodic table, in which the elements are arranged in rows and columns so that elements with similar chemical properties are grouped together. According to this arrangement, each element was assigned a number (atomic number) ranging from 1 for hydrogen to 92 for uranium. Because not all the elements were known at the time of Mendeleyev, blank spaces were left in the periodic table, each of which corresponded to a missing element. Further research, aided by the arrangement of the known elements in the chart, led to the discovery of missing elements. Elements of higher atomic number have correspondingly heavier atomic weights; this fact could have been predicted from Prout's hypothesis.

Size of the Atom

Curiosity about the size of the atom and its weight tantalized hundreds of scientists for a long period during which lack of adequate instruments and proper techniques prevented them from obtaining satisfactory answers. Subsequently, a variety of ingenious experiments was devised to determine the size and weight of the various atoms. The lightest of all atoms, hydrogen, has a diameter of 1 × 10-8 cm (0.00000001 cm) and weighs 1.7 × 10-24 (the fraction of a gram represented by 17 preceded by 23 zeros and a decimal point). An atom is so small that a single drop of water contains more than a million million billion atoms.


That the atom is not a solid bit of matter, incapable of further subdivision, became evident with the discovery of radioactivity. In 1896 the French physicist Antoine Henri Becquerel found that certain substances, such as uranium salts, give off penetrating rays of mysterious origin. Only a year earlier the German scientist Wilhelm Conrad Roentgen had announced the discovery of X rays, which can penetrate sheets of lead. The French scientists Marie Curie and her husband Pierre Curie contributed further to an understanding of radioactive substances (see Radium). As a result of the research of the British physicist Ernest Rutherford and his contemporaries, it was shown that uranium and some other heavy elements, such as thorium and radium, emit three different kinds of radiation, initially called alpha, beta, and gamma rays. The first two, which were found to consist of electrically charged bits of matter, are now called alpha and beta particles. Gamma rays eventually were identified as electromagnetic waves, similar to X rays but of shorter wavelengths.

Rutherford Nuclear Atom

Recognition of the nature of radioactive emissions enabled physicists to penetrate into the mystery of the atom. Far from being a solid bit of matter, the atom was found to consist mostly of space. At the center of this space is an infinitesimally small core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. He also proposed that satellites called electrons travel in orbits around the nucleus (see The Standard Model: Electron). The nucleus has a positive charge of electricity; the electrons each have a negative charge. The charges carried by the electrons add up to the same amount of electricity as resides in the nucleus, and thus the normal electrical state of the atom is neutral.

Bohr Atom

To explain the structure of the atom, the Danish physicist Niels Bohr developed in 1913 a hypothesis known as the Bohr theory of the atom (see Quantum Theory - Coming soon). He assumed that electrons are arranged in definite shells, or quantum levels, at a considerable distance from the nucleus. The arrangement of these electrons is called the electron configuration. The number of such electrons equals the atomic number of the atom; hydrogen has a single orbital electron, helium has 2, and uranium has 92. The electron shells are built up in a regular fashion from a first shell to a total of seven shells, each of which has an upper limit to the number of electrons that it can accommodate. The first shell is complete with two electrons, the second can hold up to eight electrons, and successive shells hold still larger numbers. The "last" electrons, those which are outermost or added last to the atom's structure, determine the chemical behavior of the atom.

It is convenient to visualize the electrons moving about the nucleus of an atom much as if they were planets moving about the sun. This view is much more precise than that held by contemporary physicists, however. It is now known that it is impossible to pinpoint the precise position of an electron in the atom's space without disturbing its predicted location at some future time. This uncertainty is resolved by attributing to the atom a cloudlike form, in which the electron's position is defined in terms of the probability of finding it at some distance from the nucleus. This rather fuzzy schematic conception of the atom may be reconciled with the solar-system model by noting that in the tiny space of the atom the electron, which makes many billions of orbits around the nucleus in a single second, is everywhere at once. The cloud view thus gives a form to the atom that is not supplied by a solar-system model.

An electrostatic force keeps the electrons from flying off. The positive charges from the nucleus hold the electrons at very precise energy levels (orbitals). A certain energy level can hold a maximum of electrons. Bohr proposed that this maximum number of electrons per energy level can be calculated mathematically by 2n2, where n is the number of the level. For example, the 3e energy level can contain just 18 electrons.

It is important to understand that electrons from lower energy levels are held in more firmly than electrons from the higher levels. However, if the atom is exposed to an exterior energy source, the electrons from inferior energy levels can jump up to a higher level. The electrons jump from one level to another when they absorb energy.

1: Electron at the fundamental level

2: A form of energy is absorbed by the electron

3: The electron jumps to a higher energy level

4: The electron returns to its fundamental state

5: The emission of electromagnetic radiation

Because this increase in energy in momentary, the electron cannot stay at a level that is too high. It will return to its fundamental level. During this return of the electron to its fundamental level, there will be an emission of radiation.

Atomic Nucleus

In 1905 Albert Einstein developed his mass-energy equation, E=mc2, as part of his special theory of relativity. This equation states that with a given mass (m) is associated an amount of energy (E) equal to this mass multiplied by the square of the velocity of light (c). A very small amount of mass is equivalent to a vast amount of energy. Because more than 99 percent of the atom's mass is in the nucleus, any release of the atom's energy would have to come from the nucleus.

In 1919 Rutherford exposed nitrogen gas to a radioactive source that emitted alpha particles. Some of the alpha particles collided with the nuclei of the nitrogen atoms. As a result of these collisions, the nitrogen atoms were transmuted into oxygen atoms. A positively charged particle was emitted from the nucleus of each of the atoms undergoing transmutation. These particles were recognized as being identical to the nuclei of hydrogen atoms. They are called protons (see Proton). Although further research proved that protons are constituents of the nuclei of all elements, no more clues to the structure of the nucleus were found until 1932, when the British physicist Sir James Chadwick discovered in the nucleus another particle, known as the neutron, having the same weight as the proton but without an electrical charge. It was then realized that the nucleus is made up of protons and neutrons. In any given atom, the number of protons is equal to the number of electrons and hence to the atomic number of the atom. Isotopes are then explained as atoms of the same element (that is, containing the same number of protons) that have different numbers of neutrons. In the case of chlorine, one isotope is identified by the symbol of 35Cl and its heavy relative by 37Cl. The superscripts identify the mass number of the isotope and are numerically equal to the total number of neutrons and protons in the nucleus of the atom. Sometimes the atomic number is given as a subscript.

The least stable arrangement of nuclei is one in which an odd number of neutrons and an odd number of protons are present; all but four isotopes containing nuclei of this kind are radioactive. The presence of a large excess of neutrons over protons detracts from the stability of a nucleus; nuclei in all isotopes of elements above bismuth in the periodic table contain this type of arrangement, and they are all radioactive. Most known stable nuclei contain an even number of protons and an even number of neutrons.

Hydrogen Isotopes

The atomic number of an atom represents the number of protons in its nucleus. This number remains constant for a given element. The number of neutrons may vary, however, creating isotopes that have the same chemical behavior, but different mass. The isotopes of hydrogen are protium (no additional neutrons), deuterium (one neutron), and tritium (two neutrons). Hydrogen always has one proton in its nucleus, which is balanced by one electron. These pictures are schematic representations of the atom. In reality the nucleus is 10,000 times smaller than the atom, and the electron is a million times smaller than the nucleus. The size of the atom is determined by the motion of the electron, which occurs in regions of space called orbitals.

Artificial Radioactivity

Experiments by the French physicists Frédérick and Irène Joliot-Curie in the early 1930s showed that stable atoms of an element may be made artificially radioactive by suitable bombardment with nuclear particles or rays. Such radioactive isotopes (radioisotopes) are produced as a result of a nuclear reaction, or transformation. In such reactions the 270-odd isotopes found in nature serve as targets for nuclear projectiles. The development of atom smashers, or accelerators, for hurling these projectile-particles to high energy has made it possible to observe thousands of nuclear reactions.

Particle Accelerators

A particle accelerator is adevices used to accelerate charged elementary particles or ions to high energies. Particle accelerators today are some of the largest and most expensive instruments used by physicists. They all have the same three basic parts: a source of elementary particles or ions, a tube pumped to a partial vacuum in which the particles can travel freely, and some means of speeding up the particles.

The big circle marks the location of the Large Electron-Positron (LEP) collider at the European particle physics laboratory in CERN. The tunnel where the particles are accelerated is located 100m (320 ft) underground and is 27 km (16.7 mi) in circumference. The smaller circle is the site of the smaller proton-antiproton collider. The border of France and Switzerland bisects the CERN site and the two accelerator rings.

Charged particles can be accelerated by an electrostatic field. For example, by placing electrodes with a large potential difference at each end of an evacuated tube, British scientists John D. Cockcroft and Ernest Thomas Sinton Walton were able to accelerate protons to 250,000 eV, (see Electron Volt). Another electrostatic accelerator is the Van de Graaff accelerator, which was developed in the early 1930s by the American physicist Robert Jemison Van de Graaff. This accelerator uses the same principles as the Van de Graaff Generator. The Van de Graaff accelerator builds up a potential between two electrodes by transporting charges on a moving belt. Modern Van de Graaff accelerators can accelerate particles to energies as high as 15 MeV (15 million electron volts).

Nuclear Forces

Modern nuclear theory is based on the notion that nuclei consist of neutrons and protons that are held together by extremely powerful "nuclear" forces. The elucidation of these nuclear forces requires physicists to disrupt neutrons and protons by bombarding nuclei with extremely energetic particles. Such bombardments have revealed more than 200 so-called elementary particles, or tiny bits of matter, most of which exist for much less than one hundred-millionth of a second.

This subnuclear world was first revealed in cosmic rays (see Cosmic Rays). These rays consist of highly energetic particles that constantly bombard the earth from outer space, penetrating down through the atmosphere and even into the earth's crust. Cosmic radiation includes many types of particles, some having energies far exceeding anything achieved in particle accelerators. When these energetic particles strike nuclei, new particles are created. Among the first such particles to be observed were the muons (detected in 1937) and pions (1947). The existence of the pion had been predicted in 1935 by the Japanese physicist Yukawa Hideki.

According to the most widely accepted theory, nuclear particles are held together by "exchange forces," in which pions common to both neutrons and protons are continuously exchanged between them. The binding of protons and neutrons by pions is similar to the binding of two atoms in a molecule through sharing or exchanging a common pair of electrons. These particles are about 200 times as heavy as electrons. The muon is essentially a heavy electron and can be either positively or negatively charged. The pion, slightly heavier than the muon, can carry a positive or negative charge, or no charge.

Elementary Particle Tracks

Particle tracks are formed by elementary particles in a bubble chamber at the CERN facility located outside of Geneva, Switzerland. By examining these tracks, physicists can determine certain properties of particles that traveled through the bubble chamber. For example, a particle's charge can be determined by noting the type of path the particle followed. The bubble chamber is placed within a magnetic field, which causes a positively charged particle's track to curve in one direction, and a negatively charged particle's track to curve the opposite way; neutral particles, unaffected by the magnetic field, move in a straight line.

Accelerator studies eventually established that each kind of particle also has an antiparticle of the same mass but opposite in charge or other electromagnetic property. Physicists have long sought a theory that would put this bewildering array of particles in order. Particles are now grouped according to the force that usually controls their interactions. Hadrons (strong nuclear force) include hyperons, mesons, and the neutron and proton. Leptons (electromagnetic and weak forces) include the tau, muon, electron, and neutrinos. Bosons (particlelike objects associated with interactions) include the photon and the hypothetical carriers of the weak force and of gravitation. The weak nuclear force is evident in such radioactive or particle-decay reactions as alpha decay (the release of a helium nucleus from an unstable atomic nucleus). (see Antimatter).

In 1963 the U.S. physicists Murray Gell-Mann and George Zweig proposed that hadrons are actually combinations of more fundamental particles called quarks, the interactions of which are carried by particlelike gluons. This theory underlies current investigations and has served to predict the existence of further particles.

Release of Atomic Energy

Two nuclear processes of great practical significance because they provide vast amounts of energy are fission, the splitting of a heavy nucleus into lighter ones, and thermonuclear fusion, the fusion of two light nuclei (at extremely high temperatures) to form a heavier one. The Italian-born American physicist Enrico Fermi achieved fission in 1934, but the reaction was not recognized as such until 1939, when the German scientists Otto Hahn and Fritz Strassmann announced that they had split uranium nuclei by bombarding them with neutrons. Neutrons are also released by the reaction and can cause a chain reaction with other nuclei. An uncontrolled chain reaction is seen in the explosion of an atomic bomb. Heat from controlled reactions, however, as in nuclear reactors, can be used to produce electric power.

Thermonuclear fusion occurs in stars, including the sun, and is the source of their heat and light. Uncontrolled fusion is seen in the explosion of a hydrogen bomb, but physicists are currently trying to develop a practical controlled-fusion device.

Target Chamber of the Nova Laser

Technicians install diagnostic equipment on the target chamber of Nova, the most powerful laser in the world. Inside the target chamber, ten laser beams are simultaneously directed toward a small fuel sample, producing fusion reactions. The laser is currently used for weapons research and should help scientists in the future explore the use of fusion as a possible energy source.

Definitions & Descriptions:


Proton, nuclear particle having a positive charge identical in magnitude to the negative charge of an electron and, together with the neutron, a constituent of all atomic nuclei. The proton is also called a nucleon, as is the neutron. The proton forms, by itself, the nucleus of the hydrogen atom. The mass of a proton is approximately 1836 times that of an electron, or 1.6726 × 10-24 g. Consequently, the mass of an atom is contained almost entirely in the nucleus. The proton has an intrinsic angular momentum, or spin, and thus a magnetic moment. In addition, the proton obeys the exclusion principle. The number of protons in the nucleus of an atom determines what element it is; the atomic number of an element denotes the number of protons in the nucleus. In nuclear physics, the proton is used as a projectile in large accelerators to bombard nuclei to produce fundamental particles (see Particle Accelerators). As the hydrogen ion, the proton plays an important role in chemistry.

The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life. Although physicists had postulated the existence of this elementary particle since the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Lawrence Berkeley National Laboratory.

Because protons are essential parts of ordinary matter, they are obviously stable. Particle physicists are nevertheless interested in learning whether protons eventually decay after all, on a time scale of many billions of billions of years. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme. Many of these attempts call for the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. By the end of the 1980s no clear evidence had yet been found; possible results thus far can also be interpreted in other ways.

Electron Volt

Electron Volt, a unit of energy used by physicists to express the energy of ions and subatomic particles that have been accelerated in particle accelerators. One electron volt is equal to the amount of energy gained by an electron traveling through an electrical potential difference of 1 V. One electron volt is equivalent to 1.60207 × 10-19 J. Electron volts are commonly expressed as million electron volts (MeV) and billion electron volts (GeV).

Cosmic Rays

Cosmic Rays, subatomic particles arriving from outer space, which have high energy as a result of their rapid motion. They were discovered when the electrical conductivity of the earth's atmosphere was traced to ionization caused by energetic radiation. The Austrian-American physicist Victor Franz Hess showed in 1911-12 that atmospheric ionization increases with altitude, and he concluded that the radiation must be coming from outer space. The discovery that the intensity of the radiation depends on latitude implied that the particles composing the radiation are electrically charged and are deflected by the earth's magnetic field.


The three key properties of a cosmic-ray particle are its electric charge, its rest mass, and its energy. The energy depends on the rest mass and the velocity. Each method of detecting cosmic rays yields information about a specific combination of these properties. For example, the track left by a cosmic ray in a photographic emulsion depends on its charge and its velocity; an ionization spectrometer determines its energy. Detectors are used in appropriate combinations on high-altitude balloons or on spacecraft (to get outside the atmosphere) to determine, for each charge and mass of cosmic-ray particle, the distribution over various energies.

About 87 percent of cosmic rays are protons (hydrogen nuclei), and about 12 percent are alpha particles (helium nuclei). Heavier elements are also present, but in greatly reduced numbers. For convenience, scientists divide the elements into light (lithium, beryllium, and boron), medium (carbon, nitrogen, oxygen, and fluorine), and heavy (the remainder of the elements). The light elements compose 0.25 percent of the cosmic rays. Because the light elements constitute only about 1 billionth of all matter in the universe, it is believed that light-element cosmic rays are formed by the fragmentation of heavier cosmic rays that collide with protons, as they must do in traversing interstellar space. From the abundance of light elements in cosmic rays, it is inferred that cosmic rays have passed through material equivalent to a layer of water 4 cm (about 1.5 in) thick. The medium elements are increased by a factor of about 10 and the heavy elements by a factor of about 100 over normal matter, suggesting that at least the initial stages of acceleration to the observed energies occur in regions enriched in heavy elements.

Energies of cosmic-ray particles are measured in units of giga (billion) electron volts (GeV) per proton or neutron in the nucleus. The distribution of proton energies of cosmic rays peaks at 0.3 GeV, corresponding to a velocity two-thirds that of light, and falls toward higher energies, although particles up to 1011 GeV have been detected through showers of secondary particles created when they collide with atmospheric nuclei. About 1 electron volt of energy per cubic centimeter of space is invested in cosmic rays in our galaxy, on the average.

Even an extremely weak magnetic field deflects cosmic rays from straight-line paths; a field of 3 × 10-6 gauss, such as is believed to be present throughout interstellar space, is sufficient to force a 1-GeV proton to gyrate with a radius of 10-6 light-year. A 1011-GeV particle gyrates with a radius of 105 light-years, about the size of the Galaxy. Thus, the interstellar magnetic field prevents cosmic rays from reaching the earth directly from their points of origin, accounting for the directions of arrival being isotropically distributed at even the highest energies.

In the 1950s, radio emission from the Milky Way galaxy was discovered and interpreted as radiation from energetic electrons gyrating in interstellar magnetic fields. The intensity of the electron component of cosmic rays, about 1 percent of the intensity of the protons at the same energy, agrees with the value inferred for interstellar space in general from the radio emission.


The source of cosmic rays is still not certain. The sun emits cosmic rays of low energy at the time of large solar flares, but these events are far too infrequent to account for the bulk of cosmic rays. If other stars are like the sun, they are not adequate sources either. Supernova explosions are responsible for at least the initial acceleration of a significant fraction of cosmic rays, as the remnants of such explosions are powerful radio sources, implying the presence of energetic electrons (see Nova, Supernova and Hypernova - Coming soon). Such observations and the known frequency of supernovas suggest that adequate energy is available from this source to balance the energy of cosmic rays lost from the Galaxy, about 1041 ergs per sec (about 1031 hp). Supernovas are believed to be the sites at which the nuclei of heavy elements are formed; so it is understandable that the cosmic rays should be enriched in heavy elements if supernovas are cosmic-ray sources. Further acceleration is believed to occur in interstellar space as a result of the shock waves propagating there. No direct evidence exists that supernovas contribute significantly to cosmic rays. Theory does suggest, however, that X-ray binaries such as Cygnus X-3 may be cosmic ray sources. In these systems, a normal star loses mass onto a companion neutron star or black hole.

Radio astronomical studies of other galaxies show that they also contain energetic electrons. The nuclei of some galaxies are far more luminous than the Milky Way in radio waves, indicating that sources of energetic particles are located there. The physical mechanism producing these particles is not known.

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