AQA A-Level Chemistry AS Paper 1, November 2021: Question 14

✅ Correct Answer

C: XeF₄

Xenon tetrafluoride has 4 bonding pairs and 2 lone pairs around the central xenon atom, resulting in a square planar molecular shape.

💡 Key Knowledge

• XeF₄: Xenon (Group 18) brings 8 electrons. Four single bonds with F use 4 electrons, leaving 4 electrons (2 lone pairs). Total = 4 bond pairs + 2 lone pairs (6 electron domains). This gives an octahedral electron-pair geometry and a square planar shape.
• NH₄⁺: Nitrogen brings 5 electrons, minus 1 for positive charge = 4 electrons. Four single bonds with H. Total = 4 bond pairs, 0 lone pairs = tetrahedral.
• SF₄: Sulfur brings 6 electrons. Four single bonds with F use 4, leaving 2 electrons (1 lone pair). Total = 4 bond pairs + 1 lone pair = seesaw.
• PCl₄⁺: Phosphorus brings 5 electrons, minus 1 for charge = 4. Four single bonds with Cl. Total = 4 bond pairs, 0 lone pairs = tetrahedral.

🧠 Exam Technique

When tackling shape multiple-choice questions:

1. Count the total number of valence electrons contributed by the central atom (remember to adjust for ionic charges!).
2. Add electrons contributed by surrounding atoms/monovalent species (each single covalent bond adds 1 electron).
3. Divide by 2 to find total electron pairs, then split into bonding pairs and lone pairs.
4. Remember that lone pairs repel more than bonding pairs and prefer positions as far apart as possible (e.g., axial positions in octahedral arrangements).

❌ Common Errors

• Forgetting to add or subtract electrons for ions (e.g., missing the positive charge adjustment on NH₄⁺ or PCl₄⁺ ).
• Confusing SF₄ (seesaw, 1 lone pair) with square planar structures.
• Assuming noble gases like Xenon cannot form bonds because they have a "full outer shell" in GCSE chemistry. At A-Level, heavier noble gases can expand their octets.