2020 O Level Chem P1 Q23 — Periodic Table / Groups and Reactivity Solution

Answer: B (P and S). Reading the Periodic Table positions, P is a Group I (alkali) metal, Q is a Group II metal, R is a metal in the central transition block, and S is a Group VII halogen. The most vigorous reaction occurs when the most reactive metal meets the most reactive non-metal, because the metal readily loses electrons while the non-metal readily gains them.

The most reactive metal available is the alkali metal P, and the only reactive non-metal is the halogen S. Their reaction, an alkali metal burning in a halogen, is very vigorous, so P and S is the answer.

The other pairs are less vigorous. Q is a Group II metal, which is less reactive than a Group I metal. R is a metal, not a reactive non-metal, so combinations P and R, and R and S, do not give the strong metal-plus-halogen reaction. The answer is B.