Spin

Spin is another name for angular momentum.

Spin is reserved in Physics for the intrinsic angular momentum of subatomic particles.

Electrons behave as though they have angular momentum ±¹/2. When an electron is bound to a proton to make up a hydrogen atom the electron may have spin-up or spin-down. The effect is quantum mechanical and has no direct analogy in classical mechanics.

The full inner shell with two electrons has one electron with spin-up and one with spin-down.

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