stable subatomic particle that has a positive charge equal in magnitude to a unit of electron charge and a rest mass of 1.67262 × 10⁻²⁷ kg, which is 1,836 times the mass of an electron.

Protons, together with electrically neutral particles called neutrons (neutron), make up all atomic nuclei except for the hydrogen nucleus (which consists of a single proton). Every nucleus of a given chemical element has the same number of protons. This number defines the atomic number of an element and determines the position of the element in the periodic table (chemical compound). When the number of protons in a nucleus equals the number of electrons orbiting the nucleus, the atom is electrically neutral.

The discovery of the proton dates to the earliest investigations of atomic structure. While studying streams of ionized (ionization) gaseous atoms and molecules from which electrons had been stripped, Wilhelm Wien (Wien, Wilhelm) (1898) and J.J. Thomson (Thomson, Sir J.J.) (1910) identified a positive particle equal in mass to the hydrogen atom. Ernest Rutherford (Rutherford, Ernest, Baron Rutherford of Nelson, of Cambridge) showed (1919) that nitrogen under alpha-particle (alpha particle) bombardment ejects what appear to be hydrogen nuclei. By 1920 he had accepted the hydrogen nucleus as an elementary particle, naming it proton.

High-energy particle-physics (physics) studies in the late 20th century refined the structural understanding of the nature of the proton within the group of subatomic particles. Protons and neutrons have been shown to be made up of smaller particles and are classified as baryons (baryon)—particles composed of three elementary units of matter known as quarks (quark).

Protons from ionized hydrogen are given high velocities in particle accelerators (particle accelerator) and are commonly used as projectiles to produce and study nuclear reactions (nuclear reaction). Protons are the chief constituent of primary cosmic rays (cosmic ray) and are among the products of some types of artificial nuclear reactions.