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For alternative meanings see proton (disambiguation).
Subatomic particle
Mass: 938.272 029(80) MeV/c2
Electric Charge: 1.602 176 53(14) × 10−19 C
Spin: 1/2
Quark Composition: 1 Down, 2 Up

In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10−19 coulomb) and a mass of 938.3 MeV/c2 (1.6726 × 10−27 kg), or about 1836 times the mass of an electron. The proton is observed to be stable, with a lower limit on its half-life of about 1035 years, although some theories predict that the proton may decay. The proton and neutron are both nucleons.

The nucleus of the most common isotope of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is.

Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign.

In chemistry and biochemistry, the term proton may refer to the hydrogen ion, H+. In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).


Ernest Rutherford is generally credited with the discover of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. Prior to Rutherford, Eugene Goldstein had observed canal rays, which were composed of positively charged ions.

Technological applications

Protons can exist in spin states. This property is exploited by nuclear magnetic resonance spectroscopy. In NMR spectroscopy, a magnetic field is applied to a substance in order to detect the shielding around the protons in the nuclei of that substance, which is provided by the surrounding electron clouds. Scientists can use this information to then construct the molecular structure of the molecule under study.


The antiproton is the antiparticle of the proton. It was discovered in the year 1955 by Emilio Segre and Owen Chamberlain, for which they were awarded a 1959 Nobel Prize in Physics.

CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10-8. The equality of their masses is also tested to better than one part in 10-8. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to 1 part in 9×10-11. The magnetic moment of the antiproton has been found with error of 8×10-3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

See also

External links


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