2021 O Level Chem P1 Q24 — Transition Elements Solution

Answer: B. Element Z forms two different chlorides, ZCl and ZCl₂, so it shows variable oxidation states (+1 and +2). It also gives a coloured solution in water. Variable oxidation states and coloured compounds are characteristic properties of transition elements. (This behaviour is that of copper: white copper(I) chloride disproportionates in water to give a blue copper(II) solution.)

Transition elements occupy the central block (the d-block) of the Periodic Table, between Group II and Group III. Letter B sits in this central transition block. Letter A is a Group I metal on the far left, while letters C and D are in the p-block on the right-hand side — so they are main-group elements, not transition elements. The answer is B.