Spin is an internal property of the particle, which behaves like angular momentum as if the particle was rotating. In fact, all elementary particles we know have spin: Leptons and quarks have spin 1/2 (in units of the Planck constant), and photon, gluon and W and Z bosons have spin 1. The Higgs boson would have zero spin, but we haven’t found it yet.

Angular momentum has a direction (it is parallel to the axis of rotation and points down if the rotation is clockwise), and so has spin. Particles with spin 1/2 have another curious property called the Paul exclusion principle, which states that you cannot have two of them in exactly the same state. If the other quantum numbers of two electrons are exactly the same, they therefore have to have opposite spin.

## Comments

strangenesscommented on :Thank you for your answer! So what does it mean when we say that electrons in a pair have “opposite spin”?

Arttucommented on :Angular momentum has a direction (it is parallel to the axis of rotation and points down if the rotation is clockwise), and so has spin. Particles with spin 1/2 have another curious property called the Paul exclusion principle, which states that you cannot have two of them in exactly the same state. If the other quantum numbers of two electrons are exactly the same, they therefore have to have opposite spin.