AQA A-Level Chemistry Paper 3, June 2018: Question 17

📌 What this question tests

This multiple-choice question tests your knowledge of periodic trends in physical properties across Period 3, specifically:

Identifying the crystal structure and bonding type of Period 3 elements (giant metallic vs simple molecular).
Factors affecting metallic bond strength (charge density and delocalised electrons).
Comparing the relative thermal energy required to overcome metallic bonding versus London (van der Waals) dispersion forces.

Question 17 · Multiple Choice (1 Mark)

Aluminium (Al) has the highest melting point among the given options (approx. 933 K / 660 °C).

Award: [1 mark] for selecting option A.

Sodium (Na): Giant metallic lattice. Each atom contributes 1 electron ( Na⁺ ions surrounded by 1 delocalised e⁻ per ion). Lower charge density means weaker metallic bonds. Melting point = 371 K.
Aluminium (Al): Giant metallic lattice. Each atom contributes 3 electrons ( Al³⁺ ions with 3 delocalised e⁻ per ion). Much higher ionic charge and smaller ionic radius lead to strong electrostatic attraction and a high melting point = 933 K.
Phosphorus (P): Simple molecular structure ( P₄ molecules held together by weak van der Waals forces). Melting point = 317 K.
Sulfur (S): Simple molecular structure ( S₈ molecules held together by van der Waals forces). Melting point = 392 K (higher than Na and P, but far below Al).

Order of melting points across the 4 options:

P (317 K) < Na (371 K) < S (392 K) < Al (933 K)

Note: Across the whole of Period 3, Silicon (Si) has the overall highest melting point (~1687 K) due to its giant covalent macromolecular network, but Silicon was not one of the choices provided.
Of the metals presented, metallic bond strength increases from Na to Mg to Al:
Al³⁺ > Mg²⁺ > Na⁺ .

Choosing Sulfur: Students often remember that sulfur ( S₈ ) has a relatively high melting point for a non-metal (higher than P₄ and even slightly higher than Na), and wrongly assume it beats Aluminium. Always remember that breaking strong metallic bonds requires vastly more energy than breaking weak intermolecular forces.
Confusing boiling and melting trends: Confusing the molecular non-metals with giant lattices. Simple molecular species ( P₄ , S₈ , Cl₂ , Ar ) only involve breaking weak intermolecular forces upon melting, not covalent bonds.
Overlooking charge on metal ions: Assuming all metals have similar melting points. Sodium only has a 1+ charge and large ionic radius, making its metallic bond relatively weak.

Classify the structure: Separate options into giant structures (Na, Al) and simple molecular structures (P, S). Simple molecular substances are held together by weak London dispersion forces and will almost always have lower melting points than metals with multi-charge ions. Eliminate B and D immediately.
Compare the remaining metals: Compare Na vs Al. Aluminium forms an Al³⁺ lattice with 3 delocalised electrons per atom, whereas Sodium forms Na⁺ with only 1 delocalised electron. The stronger electrostatic attraction in aluminium results in a substantially higher melting point. Option A is confirmed.

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