In quantum mechanics , one of two possible ways (the other being Bose-Einstein statistics ) in which a system of indistinguishable particles can be distributed among a set of energy states.
Each available discrete state can be occupied by only one particle. This exclusiveness accounts for the structure of atom s, in which electron s remain in separate states rather than collapsing into a common state. It also accounts for some aspects of electrical conductivity. This theory of statistical behaviour was developed first by P.A.M. Dirac (1926–27). The statistics apply only to particles such as electrons that have half-integer values of {{link=spin">spin ; the particles are called fermions.