Hund's Rule of Maximum Multiplicity

When electrons are being filled into a set of orbitals that all belong to the same subshell — and are therefore all equal in energy (degenerate) — Hund's rule of maximum multiplicity governs the order in which they go in: pairing of electrons within that subshell's orbitals does NOT begin until every orbital in the subshell already holds one electron each. Take the p subshell as the standard example: it has three degenerate orbitals, so the first three electrons entering a p subshell go into three SEPARATE orbitals, one electron each, all with the same (parallel) spin; only the FOURTH electron is forced to pair up, entering one of the already-singly-occupied orbitals with the opposite spin. So four electrons in a p subshell are correctly written as two orbitals singly occupied and one doubly occupied (not, incorrectly, as one orbital doubly occupied, one singly occupied and one empty). This filling pattern is not arbitrary: a subshell that ends up either exactly half-filled (one electron in every orbital) or completely filled (two electrons in every orbital) is measurably more stable than other p …