AQA A-Level Chemistry Paper 3, 2021: Question 13

1 mark · Medium difficulty · Multiple Choice

Lone Pair on Central Atom – Molecular Structures This multiple-choice question tests students’ understanding of molecular shapes and electron pair repulsion, specifically identifying a species in which the central atom retains a lone pair of electrons. It draws upon VSEPR theory and knowledge of bonding and electron arrangements in species like CO₂, SO₂, PCl₆⁻, and SO₄²⁻.

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Question

Question text

13 Which species has a lone pair of electrons on the central atom?

[1 mark]

A CO2

B SO2

C PCl ‒

D SO 2–

Mark scheme

Show the mark scheme

13 B 1 SO2

How to answer it

MCQ Analysis: Lone Pairs on the Central Atom

Correct Answer: B – SO₂

Sulfur in SO₂ has 6 valence electrons. Two are used in double bonds to each oxygen (4 electrons), leaving 2 electrons (1 lone pair) on the sulfur.

Why the Other Options Are Incorrect

A – CO₂: Carbon forms two double bonds with oxygen, using up all four of its valence electrons. It does not have a lone pair.

C – PCl₆⁻: Phosphorus expands its octet to form six P–Cl bonds, using all its valence and d-orbital electrons. No lone pairs remain on the central atom.

D – SO₄²⁻: In the sulfate ion, sulfur forms four equivalent S–O bonds. While there is resonance, no lone pair is left on the sulfur central atom.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.