Neutron, electrically uncharged elementary particle with a mass slightly greater than that of the proton. All elements except hydrogen contain neutrons in their nuclei, along with protons. Free neutrons (those not bound within a nucleus) or excess neutrons in the nucleus of heavy elements are subject to a process called beta decay. In this process the neutron decays in a proton, an electron, and an antineutrino. The neutron is a member of the baryon class of elementary particles. It was discovered by James Chadwick in 1932. Its antiparticle, the antineutron, was discovered in 1956.