Symbol s. The part of the total angular momentum of a particle, atom, nucleus, etc., that can continue to exist even when the particle is apparently at rest, i.e. when its translational motion is zero and therefore its orbital angular momentum is zero. A molecule, atom, or nucleus in a specified energy level, or a particular elementary particle, has a particular spin, just as it has a particular charge or mass. According to quantum theory, this is quantized and is restricted to multiples of h/2π, where h is the Planck constant. Spin is characterized by a quantum number s. For example, for an electron s=±½, implying a spin of+h/4π when it is spinning in one direction and –h/4π when it is spinning in the other. Because of their spin, particles also have their own intrinsic magnetic moments and in a magnetic field the spin of the particles lines up at an angle to the direction of the field, precessing around this direction. See also nuclear magnetic resonance.