proton, one of the three basic subatomic particles—along with neutrons and electrons—that make up atoms, the basic building blocks of all matter and chemistry. It is the positively charged particle that, together with the electrically neutral particles called neutrons, make up the nucleus of an atom. (The nucleus of the ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) Neutrons are slightly heavier than protons in mass, but the proton is 1,836 times heavier than the mass of an electron, which is the lightest charged particle in nature. The proton’s positive charge is equal and opposite to the negative charge on an electron, meaning a neutral atom has an equal number of protons and electrons.
More than 90 types of atoms exist in nature, and each kind of atom forms a different chemical element. Every nucleus of a given chemical element has the same number of protons, and this quantity of protons defines the atomic number of the element and determines the element’s position in the periodic table.
The discovery of the proton dates to the earliest investigations of atomic structure. While studying streams of ionized gaseous atoms and molecules from which electrons had been stripped, Wilhelm Wien (1898) and J.J. Thomson (1910) identified a positive particle equal in mass to the hydrogen atom. Ernest Rutherford showed (1919) that nitrogen under 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 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—particles composed of three elementary units of matter known as quarks.
Protons from ionized hydrogen are given high velocities in particle accelerators and are commonly used as projectiles to produce and study nuclear reactions. Protons are the chief constituent of primary cosmic rays and are among the products of some types of artificial nuclear reactions.