Spin–orbit interaction (or spin–orbit coupling) is a quantum mechanical phenomenon that arises from the interaction between a particle's intrinsic angular momentum (spin) and its orbital motion. In the context of electrons within atoms, it refers to the coupling between the electron's spin and its orbital angular momentum due to the electromagnetic effects that appear from the movement of the electron around the nucleus.
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The spin-orbit interaction couples orbital angular momentum to spin. In chemical atoms it helps determine fine structure and the coupling scale that separates weak-field Zeeman splitting from the Paschen-Back effect.