In physics, the proton (Greek πρώτον / proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10−19 coulomb), a diameter of about 1.5×10−15 m, and a mass of 938.27231(28) MeV/c2 (1.6726 × 10−27 kg), 1.007 276 466 88(13) u or about 1836 times the mass of an electron.
Protons are spin-1/2 fermions and are composed of three quarks , making them baryons. The two up quarks and one down quark of the proton are also held together by the strong nuclear force, mediated by gluons.
Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The most common isotope of the hydrogen atom is a single proton (it contains no neutrons). The nuclei of other atoms are composed of various numbers of protons and neutrons. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is.