AQA A-Level Chemistry AS Paper 1, June 2025: Question 19

AQA A-Level Chemistry • Physical Chemistry

Molecular Polarity & VSEPR Theory

What this question tests

This question assesses your ability to apply VSEPR theory (valence shell electron pair repulsion) to determine 3D molecular shapes, evaluate differences in electronegativity to identify polar bonds, and deduce whether overall molecular symmetry causes individual bond dipoles to cancel out or produce a permanent dipole moment.

Question 19 Walkthrough

Identifying the Polar Molecule from Multiple Options [1 Mark]

✅ Correct Answer

D (SCl₂)

Mark Scheme Breakdown:
• 1 mark (AO2): Selecting option D exclusively.

🧠 Exam Technique: 2-Step Polarity Rule

Check for polar bonds: Does the molecule contain bonds between atoms of different electronegativities? (All options A–D have polar bonds).
Check molecular symmetry: Do the individual dipoles cancel out?
- Symmetrical shape → Dipoles cancel → Non-polar
- Asymmetrical shape → Dipoles do not cancel → Polar

Step-by-Step Option Analysis

Deducing Electron Pairs, Shapes, and Dipole Moments

📐 Molecular Structure Breakdown

Molecule	Bonding / Lone Pairs	3D Shape	Symmetry & Net Dipole
A: BF₃	3 bonding pairs, 0 lone pairs	Trigonal planar (120°)	Symmetrical — the three equal B–F dipoles act in opposing directions and cancel out completely. Non-polar.
B: CO₂	2 double bond regions, 0 lone pairs	Linear (180°)	Symmetrical — the two equal C=O dipoles pull in directly opposite directions and cancel out. Non-polar.
C: CCl₄	4 bonding pairs, 0 lone pairs	Tetrahedral (109.5°)	Symmetrical — the four equal C–Cl dipoles cancel out in 3D space. Non-polar.
D: SCl₂	2 bonding pairs, 2 lone pairs	Bent / V-shaped (~104.5°)	Asymmetrical — sulfur has two lone pairs which distort the geometry. The dipoles do not cancel, giving a permanent net dipole.

💡 Key Knowledge: Why SCl₂ is Non-Linear

Sulfur is in Group 16 (6 valence electrons). Each Cl atom forms 1 single covalent bond, contributing 1 electron each → total 8 electrons around sulfur.
8 electrons = 4 electron pairs (2 bonding pairs + 2 lone pairs).
Lone pair–lone pair repulsion is greater than lone pair–bonding pair repulsion, which is greater than bonding pair–bonding pair repulsion.
This pushes the two S–Cl bonds down into a bent or non-linear shape (isoelectronic to H₂O). Chlorine is more electronegative than sulfur, so the bond dipoles reinforce downwards rather than canceling.

❌ Common Student Errors

Assuming 2-coordinate atoms are always linear: Students often mistakenly sketch SCl₂ as Cl—S—Cl in a straight line (confusing it with CO₂ or BeCl₂) and incorrectly conclude the dipoles cancel. Always count lone pairs on the central atom!
Confusing polar bonds with polar molecules: All four molecules possess polar covalent bonds. Polar bonds do not automatically make the overall molecule polar.
Overlooking lone pair electron repulsion: Forgetting that non-bonding electron pairs take up more space and fundamentally break molecular symmetry.