AQA A-Level Chemistry AS Paper 1, June 2022: Question 16

✅ Correct Answer: B

The oxidising ability of the element decreases.

Halogen molecules act as oxidising agents by accepting electrons:
X₂ + 2e⁻ → 2X⁻ . Down the group, the atomic radius and electron shielding increase, weakening the nuclear attraction for an incoming electron. Hence, oxidising strength decreases from F₂ to I₂.

💡 Key Knowledge: Why the Others are Incorrect

• A is incorrect: Boiling point increases down the group. Molecules have more electrons, leading to stronger induced dipole-dipole (van der Waals) forces that require more thermal energy to break.
• C is incorrect: Electronegativity decreases down the group. Fluorine is the most electronegative element (value 4.0). Larger radii and greater shielding reduce the pull on shared bonding pairs.
• D is incorrect: First ionisation energy decreases down the group because the outer shell electrons are further from the nucleus and experience more shielding.

🧠 Exam Technique: Systematic Elimination

• Identify the subject: Halogen elements (F₂, Cl₂, Br₂, I₂), not halide ions.
• Use physical states as a memory hook: F₂ and Cl₂ are gases, Br₂ is a liquid, I₂ is a solid at room temperature. This immediately proves that boiling point increases, ruling out option A.
• Recall extreme benchmarks: Fluorine is the most electronegative element and the most reactive non-metal oxidiser on the Periodic Table. Any option claiming electronegativity increases down the group must be false.

❌ Common Student Pitfalls

• Halogen vs Halide confusion: Confusing the oxidising ability of halogens (X₂) with the reducing ability of halide ions (X⁻).
  • Halogen oxidising ability decreases down the group.
  • Halide ion reducing ability increases down the group.
• Bond breaking vs Intermolecular forces: Incorrectly thinking that boiling point depends on covalent bond strength (H-X or X-X) rather than intermolecular forces between X₂ molecules.