PROTON

a stable subatomic particle that has a unit-positive charge and a mass of 1.6726231 10-27 kg, which is 1,836 times the mass of an electron. While studying streams of gas atoms and molecules from which electrons had been stripped (ionized), Wilhelm Wien (1898) and J.J. Thomson (1910) identified a positive particle equal in mass to the hydrogen atom. Ernest Rutherford showed (1919) that nitrogen under alpha particle bombardment ejects what appear to be hydrogen nuclei. By 1920 he accepted the hydrogen nucleus as an elementary particle, naming it proton. Protons, together with electrically neutral particles called neutrons, make up all atomic nuclei except for the hydrogen nucleus (which consists of a single proton). Every nucleus of a given chemical element has the same number of protons. This number relates to the atomic number of an element, which determines the position of the element in the periodic table (see periodic law). Because the number of protons in a nucleus equals the number of electrons orbiting the nucleus, under most conditions, the atom is electrically neutral. Protons are classified as baryons, particles that are made up of three smaller units of matter known as quarks (see quark). The proton has an antimatter counterpart, the antiproton, which has the identical mass but a negative electric charge. Protons from ionized hydrogen are given high velocities in particle accelerators and are commonly used as projectiles to produce and study nuclear reactions. Protons are the chief constituent of primary cosmic rays and are among the products of some types of artificial nuclear reactions.