AQA A-Level Chemistry Paper 3, June 2025: Question 11

📌 What this question tests

This question assesses knowledge from AQA Specification 3.2.4 (Properties of Period 3 elements and their oxides):

Classifying Period 3 oxides by structure and bonding: giant ionic, giant covalent (macromolecular), or simple molecular.
Linking structure and bonding types directly to physical properties (melting points).
Understanding the ionic nature of aluminium oxide (with covalent character) in the AQA syllabus classification.

Row A is the only completely correct classification:

You can solve this question in under 15 seconds by checking the distinct categories:

Identify the only macromolecular oxide: Only SiO₂ is giant covalent. This immediately rules out C (which puts Al₂O₃ here) and D (which puts MgO and Al₂O₃ here).
Check simple molecular: P₄O₁₀ and SO₃/SO₂ are simple molecules held by weak intermolecular forces. Row B incorrectly places P₄O₁₀ under giant covalent.
This leaves A as the only viable option.

Oxide	Formula	Melting Point / °C	Structure & Bonding	Why the melting point is high / low?
Sodium oxide	Na₂O	920	Giant ionic	Strong electrostatic attractions between oppositely charged ions in a giant ionic lattice require large amounts of heat energy to overcome. MgO has a higher m.p. than Na₂O due to 2+ ions having higher charge density than 1+ ions.
Magnesium oxide	MgO	2900	Giant ionic
Aluminium oxide	Al₂O₃	2040	Giant ionic (with covalent character)
Silicon dioxide	SiO₂	1610	Giant covalent (macromolecular)	Many strong covalent bonds throughout the continuous tetrahedral lattice require large amounts of energy to break.
Phosphorus(V) oxide	P₄O₁₀	580	Simple molecular	Covalent bonds hold atoms together inside the molecules, but only weak intermolecular forces (van der Waals / dipole-dipole) exist between molecules, which need little energy to overcome. P₄O₁₀ has a higher m.p. than SO₃ because it is a much larger molecule with more electrons, leading to stronger van der Waals forces.
Sulfur trioxide	SO₃	17	Simple molecular

Misclassifying Al₂O₃: Because aluminium oxide has significant covalent character (due to the polarizing power of Al³⁺) and reacts amphoterically, students often mistakenly categorize it as giant covalent. In the AQA specification, it is classified fundamentally as giant ionic.
Misinterpreting P₄O₁₀'s melting point: At 580 °C, P₄O₁₀ sublimes/melts at a relatively high temperature compared to SO₃ (17 °C), leading some students (Row B) to guess it is giant covalent. However, its structure consists of distinct, discrete P₄O₁₀ cages held by van der Waals forces.
Confusing covalent bonds with intermolecular forces: When melting P₄O₁₀ and SO₃, covalent bonds do not break—only weak van der Waals forces between molecules are broken.

Moving across Period 3:

Na₂O, MgO, Al₂O₃: Metal + Non-metal → Ionic (high melting points).
SiO₂: Metalloid + Non-metal → Giant covalent (high melting point).
P₄O₁₀, SO₃, SO₂: Non-metal + Non-metal → Simple molecular (low melting points).

Mark Scheme Breakdown: Option A = 1 Mark (AO1).

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