AQA A-Level Chemistry Paper 3, June 2025: Question 15

✅ Correct Answer

Correct Option: B

Atomic radius decreases steadily from sodium (Na) to chlorine (Cl) / argon (Ar).

• Why it's true: Across Period 3, the number of protons (nuclear charge) increases by 1 for each successive element.
• Electrons are added to the same outer quantum shell (principal energy level n = 3), so electron shielding remains roughly similar.
• The increased nuclear charge exerts a stronger electrostatic attraction on the outer electrons, pulling them closer to the nucleus.

💡 Key Knowledge: Period 3 Trends

• Atomic Radius: Decreases across the period because nuclear charge increases while shielding remains similar.
• First Ionisation Energy: Shows a general increase across the period due to greater nuclear charge and smaller radius (with minor drops at Al and S due to 3p subshell shielding and 3p spin-pairing repulsion).
• Electronegativity: Increases across the period (up to chlorine) as nuclear charge increases and bonding pairs of electrons are drawn closer to the nucleus.

🧠 Exam Technique: Elimination Strategy

On multiple-choice recall questions, quickly evaluate each option against fundamental periodicity principles:

• Option A: False. Atomic radius shows a monotonic decrease; it never increases from Na to Si. (Do not confuse this with melting points!).
• Option B: True. Matches the standard trend directly.
• Option C: False. Ionisation energy generally increases due to greater nuclear charge.
• Option D: False. Electronegativity increases towards the highly electronegative non-metals (like Cl).

❌ Common Errors & Misconceptions

• Confusing Atomic Radius with Melting Point: Students often confuse the atomic radius trend with the melting point trend (which increases from Na to Si due to metallic bonding getting stronger up to giant covalent Si, before dropping for simple molecular non-metals).
• Assuming more electrons mean bigger atoms: A common misconception is thinking that adding electrons makes an atom larger. Because extra electrons go into the same shell, the dominant factor is the increasing positive charge of the nucleus.
• Forgetting definitions: Mixing up electronegativity (ability to attract a bonding pair) with electropositivity.