Proton

In physics, the proton is a subatomic particle with a positive fundamental
electric charge of 1.6 × 10-19 coulomb, a mass of 938 MeV (1.6726231
× 10-27 kg, or about 1800 times that of an electron) and a half-life
of about 1033 years. 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.

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 be 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).

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