The Physics
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Diameter of a Proton

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Sears, W. Francis. University Physics. New York. Addison Wesley, 1992: 596-603. "Our physics books say that the diameter of a proton is 2 × 10−14 me [sic]" 2 × 10−14 m
Christensen, James J. The Structure of an Atom. London: Wiley, 1990: 60-65. "The structure is reflected in the size of nucleons which are about 10−15 m across" 10−15 m
MIllikan, Robert Andrews. Electronics (+ and -) Protons, Photons, Neutrons, and Cosmic Rays. London: Cambridge University Press, 1990: 47. "The radius of a proton is on the order of 10−13 cm" 10−15 m
Brown, Jonathan. The Physical Science Encyclopedia. New York. Cornell University Press, 1980. "The proton has a radius about 10−15 m" 10−15 m
World Book Encyclopedia. Chicago: World Book, 1998: 69. "A proton has a diameter of approximately one-millionth of a nanometer" 10−15 m

A proton is a nuclear particle that has a positive charge.This charge is equal in magnitude to that of an electron, but opposite in sign. Protons together with neutrons constitute all atomic nuclei. Protons and neutrons together are regarded as nucleons. The nuclear forces operating between a neutron and a proton are much stronger than the electrostatic forces existing between atomic and and molecular systems. The nuclear force between two nucleons is considered separately from the electrostatic forces due to electric charges that nucleons could carry. In addition mass, charge, spin, and diameter play an important role in the mechanics of the nucleus.

Protons are the nuclei of the simplest element -- hydrogen. Protons also differentiate one element from the other because, every different element has a different number of protons. Positively charged hydrogen atoms were first identified as protons by J. J. Thompson in 1906. He found that the electric charge on a proton was equal but opposite that of an electron. However, the mass of a proton he found to much larger than that of an electron.

The current estimate for the size of a proton was shown by Rutherford in his scattering experiment in 1911. Rutherford showed that the nucleus of the atom is very small and dense compared to the rest of the atom. Its diameter was later found to be on the order of 10−15 m.

The angular pattern and energy distribution of scattered electrons gives us information about the internal structure of protons. In 1963, M. Gell-Mann proposed that a proton was composed of three spinning particles called quarks: two up quarks with a charge of +2/3 e (positive two-thirds the electron's charge) and one down quark with a charge of −1/3 e. This fractionally charged quark concept was developed much further and has become central to understanding the behavior and structure of protons.

Yelena Meskina -- 1999