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elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons. Every atomic nucleus contains one or more protons; the number of protons, called the atomic number, is different for every element (see periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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). The mass of the proton is about 1,840 times the mass of the electron and slightly less than the mass of the neutronneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
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. The total number of nucleons, as protons and neutrons are collectively called, in any nucleus is the mass number of the nucleus. The existence of the nucleus was postulated by Ernest Rutherford in 1911 to explain his experiments on the scattering of alpha particles; in 1919 he discovered the proton as a product of the disintegration of the atomic nucleus. The proton and the neutron are regarded as two aspects or states of a single entity, the nucleon. The proton is the lightest of the baryonbaryon
[Gr.,=heavy], class of elementary particles that includes the proton, the neutron, and a large number of unstable, heavier particles, known as hyperons. From a technical point of view, baryons are strongly interacting fermions; i.e.
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 class of elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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. The proton and other baryons are composed of triplets of the elementary particle called the quark. A proton, for instance, consists of two quarks called up and one quark called down, a neutron consists of two down quarks and an up quark. The antiparticleantiparticle,
elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
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 of the proton, the antiproton, was discovered in 1955; it has the same mass as the proton but a unit negative charge and opposite magnetic moment. Protons are frequently used in a particle acceleratorparticle accelerator,
apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies of its
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 as either the bombarding (accelerated) particle, the target nucleus, or both. The possibility that the proton may have a finite lifetime has recently come under examination. If the proton does indeed decay into lighter products, however, it takes an extremely long time to do so; experimental evidence suggests that the proton has a lifetime of at least 1031 years.


A positively charged particle that is the nucleus of the lightest chemical element, hydrogen. The hydrogen atom consists of a proton as the nucleus, to which a single negatively charged electron is bound by an attractive electrical force (since opposite charges attract). The proton is about 1836 times heavier than the electron, so that the proton constitutes almost the entire mass of the hydrogen atom. Most of the interior of the atom is empty space, since the sizes of the proton and the electron are very small compared to the size of the atom. See Atomic structure and spectra

For chemical elements heavier than hydrogen, the nucleus can be thought of as a tightly bound system of Z protons and N neutrons. An electrically neutral atom will then have Z electrons bound comparatively loosely in orbits outside the nucleus. See Neutron, Nuclear structure

The numerical values of some overall properties of the proton can be summarized as follows: charge, 1.602 × 10-19 coulomb; mass, 1.673 × 10-27 kg; spin, (½)ℏ (where ℏ is Planck's constant h divided by 2&pgr;); magnetic dipole moment, 1.411 × 10-26 joule/tesla; radius, about 10-15 m. See Fundamental constants, Nuclear moments, Spin (quantum mechanics)

It is instructive to contrast the proton's properties with those of the electron. All of the electron's properties have been found to be those expected of a spin-½ particle which is described by the Dirac equation of quantum mechanics. Such a Dirac particle has no internal size or structure. See Electron, Relativistic quantum theory

By contrast, although it also has a spin of ½, the proton's magnetic moment, which is different from that for a Dirac particle, and its binding with neutrons into nuclei strongly suggest that it has some kind of internal structure, rather than being a point particle. Two different kinds of high-energy physics experiments have been used to study the internal structure of the proton. An example of the first type of experiment is the scattering of high-energy electrons, above say 1 GeV, from a target of protons. The angular pattern and energy distribution of the scattered electrons give direct information about the size and structure of the proton. The second type of high-energy experiment involves the production and study of excited states of the proton, often called baryonic resonances. It has been found that the spectrum of higher-mass states which are produced in high-energy collisions follows a definite pattern. See Baryon

In 1963, M. Gell-Mann and, independently, G. Zweig pointed out that this pattern is what would be expected if the proton were composed of three spin-½ particles, quarks, with two of the quarks (labeled u) each having a positive electric charge of magnitude equal to ⅔ of the electron's charge (e), and the other quark (labeled d) having a negative charge of magnitude of ⅓e. Subsequently, the fractionally charged quark concept was developed much further, and has become central to understanding every aspect of the behavior and structure of the proton. See Quarks

An important class of fundamental theories, called grand unification theories (GUTs), makes the prediction that the proton will decay. The predicted lifetime of the proton is very long, about 1030 years or more—which is some 1020 times longer than the age of the universe—but this predicted rate of proton decay may be detectable in practical experiments. See Grand unification theories

If the proton is observed to decay, this new interaction will also have profound consequences for understanding of cosmology. The very early times of the big bang (about 10-30 s) are characterized by energies so high that the same grand unified interaction which would allow proton decay would also completely determine the subsequent evolution of the universe. This could then explain the remarkable astrophysical observation that the universe appears to contain only matter and not an equal amount of antimatter. See Elementary particle


(proh -ton) Symbol: p. An elementary particle (a hadron) that has a positive charge, equal in magnitude to that of the electron. It forms the nucleus of the hydrogen atom and is present in differing numbers in all nuclei. The rest mass of the proton is 1.6726 × 10–24 gram, approximately 1836 times that of the electron. It has spin ½. It has a lifetime known to exceed 1031 years. The antiparticle of the proton is the antiproton. See also hydrogen; proton-proton chain reaction.



a stable elementary particle, the nucleus of the hydrogen atom. The proton has a mass mp = (1.6726485 ± 0.0000086) × 10-24g(or mp ≈ 1836me ≈ 938.3 megaelectron volts [MeV]/c2, where me is the mass of an electron and c is the speed of light) and a positive electric charge e = (4.803242 ± 0.000014) × 10-10 electrostatic unit. The spin of the proton is ½ (in units of Planck’s constant ), and as a particle with half integral spin, the proton obeys Fermi-Dirac statistics; that is, it is a fermion. The magnetic moment of the proton μp. = (2.7928456 ± 0.0000011)μn, where μn is the nuclear magneton. Together with neutrons, protons form the atomic nuclei of all chemical elements. The number of protons in the nucleus is equal to the atomic number and therefore determines an element’s position in the periodic table of the elements. Free protons form the main part of primary cosmic rays. The proton has an antiparticle—the antiproton.

The idea of the proton was put forward in the second decade of the 20th century in the form of a hypothesis that all nuclei consist of hydrogen atom nuclei. In 1919–20, E. Rutherford experimentally observed hydrogen nuclei ejected from the nuclei of other elements by alpha particles; in the early 1920’s he introduced the term “proton.” The observation that the atomic numbers of the elements are smaller than their atomic masses remained a puzzle until 1932, when the neutron was discovered.

The proton is a hadron, or strongly interacting particle, and is classified among the fermion hadrons, or baryons; the baryon charge of the proton is B = + 1. The principle of the conservation of baryon charge accounts for the stability of the proton, which is the lightest baryon. Protons also participate in all other types of fundamental elementary particle interactions, namely, electromagnetic, weak, and gravitational interactions.

In strong interactions, the proton and neutron have identical properties and therefore are considered as two quantum states of a single particle—the nucleon. The possibility of combining hadrons of this type into families of particles with common properties—isotopic-spin multiplets—is taken into account by the introduction of the quantum number known as isotopic spin. The isotopic spin of the nucleon is I = ½. The most important example of a strong interaction involving protons is that of the nuclear forces binding nucleons in the nucleus. The experimental investigation of strong interactions is based largely on experiments involving the scattering of protons and mesons by protons. From these experiments, new strongly interacting particles—the antiproton, hyperons, and resonances—have been discovered. It is difficult to give a theoretical explanation of the properties of the proton because a satisfactory theory of strong interactions is lacking. A general approach, which gives only a qualitative explanation, is the hypothesis that the proton is surrounded by a “cloud” of virtual particles that the proton continuously emits and absorbs. The strong interactions of protons with other particles are considered as processes of exchange of virtual hadrons.

The proton’s electromagnetic properties are closely linked to the proton’s participation in the more intensive strong interactions. An example of this link is the photoproduction of mesons, which may be considered as the ejection of mesons from the cloud of virtual hadrons surrounding protons by a gamma-quantum of energy of the order of 150 MeV or more. The interaction of protons with virtual π+-mesons provides a qualitative explanation for the large difference between the magnetic moment of the proton and that of the nuclear magneton (the two would be equal if the proton’s magnetic moment were restricted solely to a quantum mechanical description based on the Dirac equation). In the 1950’s, experiments on the scattering of electrons and gamma-quanta by protons conducted by the American physicist R. Hofstadter and others revealed a spatial distribution of the electric charge and magnetic moment of the proton, which indicated the presence of an internal structure. The effect of this “spreading” of charge and magnetic moment on the interaction of protons with electrons is usually taken into account by introducing the electrical and magnetic form factors—factors whose squares characterize the change in the cross section of scattering by an actual proton compared to the scattering by a point particle, that is, a particle with a point charge e and a point magnetic moment μp. The data obtained on the inelastic scattering of electrons with energies up to 21 gigaelectron volts (GeV) by protons apparently means that pointlike scattering centers (partons) exist in the proton.

The intranuclear transformation of a proton into a neutron, and vice versa (the beta decay of nuclei and K-capture), are examples of weak interactions involving protons. In 1953, the process opposite to beta decay was observed—the formation of a neutron and positron on absorption of an antineutrino by a free proton. This was the first direct experimental proof of the existence of neutrinos.

Because of the stability and electric charge of the proton and the relative simplicity of proton production through the ionization of hydrogen, beams of accelerated protons are one of the basic tools of experimental elementary particle physics. Protons—free (hydrogen) or bound in nuclei—are also used quite often as targets in particle collision experiments. The largest proton accelerators are the 76-GeV Serpukhov accelerator in the USSR and the 400-GeV accelerator at Batavia, III. The maximum equivalent energy on collision of protons of about 1,500 GeV was reached in an accelerator with colliding proton beams (each with an energy of 28 GeV) at the European Council for Nuclear Research (CERN) in Switzerland. Accelerated protons are used not only to study the scattering of protons themselves but also to produce beams of other particles: π-mesons, K-mesons, antiprotons, and muons. By 1973, encouraging results on the use of accelerated proton beams in medicine (radiotherapy) had been obtained.


Rutherford, E. Izbr. nauchnye trudy, book 2: Stroenie atoma i iskusstvennoe prevrashchenie elementov. Moscow, 1972. (Translated from English.)
Beiser, A. Osnomye predstavleniia sovremennoi fiziki. Moscow, 1970. (Translated from English.)
Barger, V. D., and D. B. Cline. “Rasseianie pri vysokikh energiiakh.” In the collection Elementarnye chastitsy, vol. 9. Moscow, 1973.
Kendall, H. W., and W. K. Panofsky. “Struktura protona i neitrona.” Ibid.
Gol’din, L. L. [et al.]. “Primenenie tiazhelykh zariazhennykh chastits vysokoi energii v meditsine,” Uspekhi fizicheskikh nauk, 1973, vol. 110, issue 1, pp. 77–99.




the name of a series of Soviet heavy artificial earth satellites carrying scientific research equipment for the study of cosmic rays and the interaction of superhigh-energy particles with matter.

Proton 1 was launched on July 16, 1965, Proton 2, on Nov. 2, 1965, and Proton 3, on July 6, 1966. Each Proton satellite weighed 12.2 tons, including the equipment located in the last stage of the launch vehicle; the scientific apparatus weighed 3.5 tons. The orbits had a perigee of 190 km and an apogee of approximately 630 km. The scientific apparatus included an ionization calorimeter for the study of particles with energies up to 1013 electron volts. Proton 4 was launched Nov. 16, 1968. It was equipped with unique scientific apparatus that made it possible to expand the range of energies to be studied to 1015 electron volts. Proton 4 weighed approximately 17 tons, excluding the last stage of the launch vehicle, and the scientific apparatus weighed 12.5 tons. The orbit had a perigee of 255 km and an apogee of 495 km. Satellites in the Proton series were used to study the energy spectrum and chemical composition of particles of primary cosmic rays and the intensity and energy spectrum of gamma rays and galactic electrons.

The Proton satellites were launched by powerful multistage multiengine launch vehicles. The total maximum effective power of the propulsion systems was greater than 44 gigawatts, or 60 million hp. The Proton launch vehicle was distinguished by high operating and energy parameters—basically a function of the powerful liquid-propellant rocket engines, which operated in conjunction with the combustion of producer gas. The creation of powerful, small-scale engines was made possible by high pressure in the engine system, high combustion efficiency, and a high level of expansion and a uniform and balanced outflow of combustion products from exhaust nozzles.


An elementary particle that is the positively charged constituent of ordinary matter and, together with the neutron, is a building block of all atomic nuclei; its mass is approximately 938 megaelectronvolts and spin ½.


a stable, positively charged elementary particle, found in atomic nuclei in numbers equal to the atomic number of the element. It is a baryon with a charge of 1.602176462 × 10--19 coulomb, a rest mass of 1.672 62159 × 10--27 kilogram, and spin ½


A home computer made by Acorn Computers under a contract won from the BBC in April 1981.


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