In quantum mechanics, spin is an intrinsic property of all elementary particles. Fermions, the particles that constitute ordinary matter, have half-integer spin. Spin-Â½ particles constitute an important subset of such fermions. All known elementary fermions have a spin of Â½.
Particles having spin Â½ include the proton, neutron, electron, and quarks. The dynamics of spin-Â½ objects cannot be accurately described using classical physics; they are among the simplest systems which require quantum mechanics to describe them. As such, the study of the behavior of spin-Â½ systems forms a central part of quantum mechanics.