2020 O Level Chem P1 Q7 — Atomic Structure / Ions and Electronic Structure Solution

Answer: B (\(^{32}_{16}\text{Q}\), sulfur). The electronic structure 2,8,8 means the ion has \(2 + 8 + 8 = 18\) electrons. The ion carries a 2− charge, which means it gained two electrons, so the neutral atom must have had \(18 - 2 = 16\) electrons and therefore 16 protons. An element with proton number 16 is sulfur, which readily forms the sulfide ion S²⁻.

Checking the options: \(^{16}_{8}\text{P}\) has 8 protons (oxygen) — it does form O²⁻, but it is the wrong proton number for a 2,8,8 ion. \(^{32}_{16}\text{Q}\) has 16 protons, so its 2− ion has 18 electrons arranged 2,8,8 — correct. \(^{40}_{18}\text{R}\) is argon, a noble gas that does not form ions. \(^{40}_{20}\text{S}\) is calcium, which forms a 2+ ion, not 2−. The answer is B.