AQA A-Level Chemistry Paper 3, 2024: Question 19

✅ Correct answer (what to write/tick)

D — Chlorine has the highest first ionisation energy (among Na to Cl).

Marks: 1/1 for selecting D. No working is required, but reasoning helps avoid traps.

💡 Key knowledge (why D is correct)

• First ionisation energy generally increases across a period because nuclear charge increases while electrons are added to the same shell, so shielding is similar and attraction increases.
• So, moving Na → Cl, the outer electron is held more strongly, making it harder to remove.
• Within Na to Cl, the highest first ionisation energy is at the right-hand end: Cl.

🧠 Exam technique (how to eliminate options fast)

1. Atomic radius trend: decreases across the period → smallest radius is near Cl (not Na). So A is wrong.
2. Melting point trend: rises Na → Al (stronger metallic bonding), then peaks at Si (giant covalent), then drops for P, S, Cl (simple molecular). So B is wrong.
3. Electronegativity trend: increases across the period → Cl is most electronegative (not S). So C is wrong.
4. Only statement left consistent with the overall trend is D.

This is exactly what “AO2” means here: using trends to decide, not recalling a single isolated fact.

❌ Common errors (examiner-style traps)

• Choosing B (Al highest melting point) because “metals have high melting points.” The peak in Period 3 is silicon due to a giant covalent structure.
• Choosing C (S most electronegative) because students confuse “more non-metallic” with a specific element. Across Period 3, electronegativity increases to Cl.
• Choosing A by mixing up trends: atomic radius decreases across the period; sodium is one of the largest.
• Overthinking ionisation energy exceptions (e.g., Al vs Mg, S vs P). These exceptions exist, but they don’t stop Cl being the highest among Na to Cl.