1.3.5—Electronic configurations

In a ground-state sub-shell, orbitals of the same sub-shell have equal energy. Electrons occupy these orbitals singly before any pairing, and the single electrons have parallel spins. This is Hund’s rule.

Once every equivalent orbital contains one electron, additional electrons pair in the orbitals. The two electrons in a paired orbital must have opposite spins; this arrangement limits the effect of electron–electron repulsion within the sub-shell.

Apply the rule by drawing one box per orbital: p³ is ↑, ↑, ↑, whereas p⁴ is ↑↓, ↑, ↑. The arrows show spin and the number of unpaired electrons, not merely the total electron count.

Do not pair electrons in the first orbital while equivalent orbitals remain empty, and do not give paired electrons the same spin. Keep Hund’s rule for equal-energy orbitals distinct from the separate order in which sub-shells are filled.