2019 O Level Chem P1 Q27 — Periodic Table / Properties of Elements Solution

Answer: B. Match each property to the right element.

Statement 1, proton number 24: this is chromium, Cr, element 24. Statement 2, cuts easily with a knife: this describes sodium, a soft Group I metal. Statement 3, a constituent of brass: brass is an alloy of copper and zinc, so this is zinc. Statement 4, burns in oxygen with a bright white light: this is the classic magnesium flame, \(2\text{Mg} + \text{O}_2 \to 2\text{MgO}\). Statement 5, catalyst in the Haber process: iron is the catalyst for \(\text{N}_2 + 3\text{H}_2 \to 2\text{NH}_3\).

So sodium goes with statement 2, zinc with 3, iron with 5, chromium with 1 and magnesium with 4. The row that matches every element to its correct property is row B. The common error is to confuse the soft, knife-cuttable Group I metal (sodium) with the harder transition metals, or to mix up which metal catalyses the Haber process.