The proton is a subatomic particle with a positive fundamental electric charge of 1.6 × 10^-19 coulomb. The nucleus of the most common isotope of the hydrogen atom, H, is a single proton. The nuclei of other atoms are composed of neutrons and protons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and what chemical element it is.

A proton has a mass of 1.6726231 × 10^-27 kg, which is about 1800 times heavier than an electron.

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.

Because the electromagnetic force is many orders of magnitude stronger than the gravitational force, the charge on the proton must equal to the charge on the electron, otherwise the net repulsion of having an excess of positive or negative charge (depending on which charge was numerically greater - atoms would not be electrically neutral) would cause a noticeable expansion effect on the universe, and indeed any gravitationally aggregated matter (planets, stars, etc.). It is taken that the positron (antielectron) has the same magnitude charge as the electron but opposite in sign; the same applies for the antiproton and proton.

In chemistry and biochemistry, the term proton may refer to the hydrogen ion in aqueous solution (in other words, the hydronium ion). In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).

See also: particle physics, chemistry, proton decay, neutron, proton-proton chain, proton pump inhibitor.

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