proton, elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons. Every atomic nucleus contains one or more protons; the number of protons, called the atomic number, is different for every element (see periodic table). The mass of the proton is about 1,840 times the mass of the electron and slightly less than the mass of the neutron. The total number of nucleons, as protons and neutrons are collectively called, in any nucleus is the mass number of the nucleus. The existence of the nucleus was postulated by Ernest Rutherford in 1911 to explain his experiments on the scattering of alpha particles; in 1919 he discovered the proton as a product of the disintegration of the atomic nucleus. The proton and the neutron are regarded as two aspects or states of a single entity, the nucleon. The proton is the lightest of the baryon class of elementary particles. The proton and other baryons are composed of triplets of the elementary particle called the quark. A proton, for instance, consists of two quarks called up and one quark called down, a neutron consists of two down quarks and an up quark. The antiparticle of the proton, the antiproton, was discovered in 1955; it has the same mass as the proton but a unit negative charge and opposite magnetic moment. Protons are frequently used in a particle accelerator as either the bombarding (accelerated) particle, the target nucleus, or both. The possibility that the proton may have a finite lifetime has recently come under examination. If the proton does indeed decay into lighter products, however, it takes an extremely long time to do so; experimental evidence suggests that the proton has a lifetime of at least 1031 years.