Neutron See also: Elementary Particles Uncharged Subatomic Particle discovered by Chadwick, James in 1932, of slightly greater Mass than the Proton. The stable Isotopes of all Chemical Elements (see Element) except Hydrogen and Helium contain within the nucleus a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A neutron bound within the nucleus may be stable. A nucleus with an excess of neutrons, however, is radioactive; the extra neutrons (as well as any free neutrons not bound within a nucleus) convert by beta decay (see Radioactivity) into a proton, an Electron, and an antineutrino (see Neutrino). The antineutron, the neutron's Antiparticle (see Antimatter), was discovered in 1956. The neutron is made up of still smaller particles called Quarks.