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 anatom. (The nucleus of the ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) Neutrons are slightly heavier than protons inmass, but the proton is 1,836 times heavier than the mass of anelectron, which is the lightest charged particle in nature. The proton’s positivecharge 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 ofionized gaseous atoms and molecules from which electrons had been stripped,Wilhelm Wien (1898) andJ.J. Thomson (1910) identified a positive particle equal in mass to thehydrogen atom.Ernest Rutherford showed (1919) that nitrogen underalpha-particle bombardment ejects what appear to be hydrogen nuclei. By 1920 he had accepted the hydrogen nucleus as an elementary particle, naming it proton.

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High-energyparticle-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 asbaryons—particles composed of three elementary units of matter known asquarks.

Protons from ionized hydrogen are given high velocities inparticle accelerators and are commonly used as projectiles to produce and studynuclear reactions. Protons are the chiefconstituent of primarycosmic rays and are among the products of some types of artificial nuclear reactions.