Hund's Rule
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State
Hund's rule states electrons fill degenerate orbitals singly, with parallel spins, before any orbital in that set gets a second electron.
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Nitrogen's three 2p electrons each occupy a separate 2p orbital with parallel spin, rather than pairing up in fewer orbitals.
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Pairing electrons early, before every orbital in the set holds one, violates the arrangement Hund's rule predicts.