spin

Amount of angular momentum associated with a subatomic particle or nucleus. It is measured in multiples of {Planck constant} (h-bar), equal to Planck's constant divided by 2. Electrons, neutrons, and protons have a spin of , for example, while pions and helium nuclei have zero spin. The spin of a complex nucleus is the vector sum of the orbital angular momentum and intrinsic spins of the constituent nucleons. For nuclei of even mass number, the multiple is an integral; for those of odd mass number, it is a half-integer. See also Bose-Einstein statistics, Fermi-Dirac statistics.