spin quantum number (s)

The spin quantum number, written s, is a single fixed number that labels how much intrinsic spin a kind of particle carries. It is a permanent fingerprint of the particle type: every electron has s equal to one half, every photon has s equal to one, and nothing can change that value. From s you can read off the size of the spin and how many distinct orientations it can take.

The number s is allowed only special values: zero, a half, one, three halves, and so on, in steps of a half. For a given s the spin can point in two s plus one distinguishable ways along any axis, so spin-½ has two settings, spin-1 has three, and a spinless particle has just one. This counting is what shows up directly in experiments like Stern–Gerlach, where the number of beam spots equals the number of allowed orientations.

Whether s is a whole number or a half-odd number splits all particles into two great families with utterly different social habits. Half-integer spins are fermions, which refuse to share a quantum state and so build up the structure of matter; integer spins are bosons, which happily pile into the same state and carry forces. A single small number thus decides whether a particle stacks up like bricks or merges like light.