According to Hund's rule, electrons entering a set of degenerate orbitals will
- A. pair up in the first orbital before entering the next
- B. occupy separate orbitals with parallel spins before pairing
- C. occupy the orbital of highest energy first
- D. always pair with opposite spins immediately
Explanation
Electrons repel one another, so they spread across orbitals of equal energy singly and with parallel spins before any orbital takes a second electron. This is why nitrogen, with three 2p electrons, has three unpaired electrons rather than one pair and one single. Filling lowest energy orbitals first is the Aufbau principle, a separate rule.
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