Paper 1 Q2 — Unpaired electrons in phosphorus

Atomic structure — filling p orbitals singly first (Hund’s rule)

What the examiners said

“Some confusion was shown by those who selected answers based on either the single occupation of s and p orbitals in the outer shell before pairing, or that a p orbital contains two electrons before additional electrons occupy other p orbitals.”

Try the question

How many unpaired electrons are there in a ground-state phosphorus atom (Z = 15)?

Examiner report: “Wrong answers came from pairing p electrons too early instead of filling singly first.”

Explore — fill the orbitals

Add electrons one at a time and watch Hund’s rule: within a subshell they go in singly (parallel spins) before pairing. Stop at Z = 15 to see phosphorus.

Electrons (Z): 15

Sweep to N (Z=7) or P (Z=15): three half-filled p orbitals, three unpaired electrons. Pairing only begins once every orbital in the subshell holds one electron.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Chemistry 8873 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org