2023 O Level Chem P1 Q29 — Periodic Table Solution

Answer: C. P is at the top of Group II (a small, reactive alkaline earth metal), Q is at the top right of Group 0 (a noble gas, e.g. He or Ne), R is a halogen (Group VII) and S is at the bottom of Group I (a very reactive alkali metal).

C — R (Group VII) gains one electron to form an anion R⁻; S (Group I) loses one electron to form a cation S⁺. So R does form an anion when it reacts with S. ✓

A — wrong: metallic character increases down a group and to the left across a period. S (bottom of Group I) has the greatest metallic character here, not P.
B — wrong: Q is a noble gas (Group 0). Noble gases are essentially inert in O-Level chemistry and do not react with metals to form compounds like \(\text{PQ}_2\).
D — wrong: S is in Group I and has 1 valence electron — the smallest possible for a main-group element.