proton, subatomic particle having a positive electrical charge e⁺ and constituting the nucleus of the ordinary hydrogen atom. Every atomic nucleus contains one or more protons. The mass of the proton is 1,836 times the mass of the electron and slightly less than the mass of the neutron. Protons are no longer considered elementary particles, but are made up of smaller particles called quarks. In 1919 Ernest Rutherford discovered the proton as a product of the disintegration of the atomic nucleus. The antiproton, the proton's antiparticle, was discovered in 1955. When found in the nucleus of an atom, protons and neutrons are called nucleons. Neutrons and protons belong to a class of particles called baryons which are made up of three quarks. Hadrons are any particles composed of quarks and which take part in the strong interaction, so a proton is also a hadron.