Answer: A. Aluminium is in Group III → \(\text{Al}^{3+}\). Phosphorus is in Group V → it gains 3 electrons to reach a noble-gas configuration → \(\text{P}^{3-}\) (phosphide).
Combining \(\text{Al}^{3+}\) and \(\text{P}^{3-}\) by charge balance: \(+3\) and \(-3\) cancel one-for-one, so the simplest formula is AlP. Option A.
Option B (\(\text{Al}_2\text{P}_3\)) implies \(\text{Al}^{2+}\) and \(\text{P}^{3-}\) — but aluminium doesn't form a +2 ion in Group III chemistry.
Option C (\(\text{Al}_3\text{P}_2\)) would imply \(\text{Al}^{2+}\) and \(\text{P}^{3-}\) with reversed roles, equally impossible.
Option D (\(\text{Al}_3\text{P}_5\)) doesn't correspond to any sensible charge balance.
Every O Level Pure Chemistry TYS MCQ solution video, plus full Sec 3 / Sec 4 topic recordings.
Find out more