AQA A-Level Chemistry AS Paper 1, June 2025: Question 11

1 mark · Easy difficulty · Multiple Choice

Identify the diagram showing the correct arrangement and relative sizes of ions in solid sodium chloride.

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Question

Question 11 asks: 'Which diagram best shows the arrangement and the relative sizes of the ions in solid sodium chloride?' Four diagrams labeled A to D depict four touching spherical ions arranged in a 2x2 grid. In A, the negative ions are larger than the positive ions, arranged with alternating charges. In B, the positive ions are larger than the negative ions, with alternating charges. In C, all four ions are equal in size, with negative ions on the left and positive ions on the right. In D, all four ions are equal in size, arranged with alternating charges.
Question text

11 Which diagram best shows the arrangement and the relative sizes of the ions in

solid sodium chloride?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table showing question number 11 with correct answer A, awarded 1 mark under assessment objective AO2, displaying a small diagram of option A where chloride ions are larger than sodium ions with alternating positive and negative charges.

11 A 1 (AO2)

How to answer it

Structure and Ionic Radii in Solid Sodium Chloride

What this question tests

This question evaluates your understanding of two core physical chemistry principles:

  • Periodic trends in ionic radii: Comparing the physical size of cations (Na⁺) and anions (Cl⁻) based on electron configuration and shielding.
  • Giant ionic lattice arrangement: How oppositely charged ions alternate in regular 3D crystal structures to maximise electrostatic attractions and minimise repulsions.
Question 11

Giant Ionic Crystal Lattice & Relative Ion Sizes

AQA A-Level Chemistry • Paper 1 Multiple Choice • 1 Mark (AO2)

✅ Correct Answer: A

Diagram A is the correct representation because:

  • Correct relative size: The chloride ion (Cl⁻) is noticeably larger than the sodium ion (Na⁺).
  • Correct alternating arrangement: Each positive ion is surrounded by and adjacent to negative ions, and vice-versa, avoiding like-charge repulsions.

📋 Mark Scheme Breakdown

Key: A
Mark allocation: 1 mark (AO2 - application of knowledge to models)
Examiner note: Requires dual deduction: relative ion radius (Cl⁻ > Na⁺) AND alternating charge packing.

💡 Key Knowledge: Why is Cl⁻ larger than Na⁺?

  • Electronic configurations:
    • Na atom is 1s² 2s² 2p⁶ 3s¹ → loses 1 electron to form Na⁺ ( 1s² 2s² 2p⁶ ). It has only 2 occupied quantum shells.
    • Cl atom is 1s² 2s² 2p⁶ 3s² 3p⁵ → gains 1 electron to form Cl⁻ ( 1s² 2s² 2p⁶ 3s² 3p⁶ ). It retains 3 occupied quantum shells.
  • Shells & Shielding: Because Cl⁻ has electrons in the third quantum energy level (n = 3), its outer electrons experience greater shielding from inner shells and are further from the nucleus, giving Cl⁻ a significantly larger ionic radius (~181 pm) compared to Na⁺ (~102 pm).
  • Lattice Geometry: In solid NaCl (a face-centred cubic giant lattice), each Na⁺ is octahedrally coordinated by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions (coordination number 6:6). In any 2D plane through the lattice, positive and negative charges must strictly alternate.

🧠 Exam Technique: Systematic Elimination

For multiple choice questions featuring geometric representations:

  1. Filter 1: Eliminate wrong sizes
    Cl⁻ has 3 shells; Na⁺ has 2 shells. Therefore, the negative sphere must be larger than the positive sphere.
    → Eliminates B (where positive is larger) and C & D (where both spheres are identical in size).
  2. Filter 2: Verify lattice coordination
    Adjacent touching ions along the lattice axes must have opposite charges to ensure strong net electrostatic attraction.
    → Confirms A and additionally rules out C (which incorrectly places like charges adjacent to one another).

❌ Common Pitfalls & Misconceptions

  • Confusing atomic radius with ionic radius: Students remember that atomic radius decreases across a period, but forget that Na⁺ has lost its entire 3s outer shell, leaving it with only 2 shells compared to Cl⁻'s 3 shells.
  • Assuming ions are equal in size (Options C & D): Simple ball-and-stick diagrams in textbooks often use uniform sphere sizes for clarity, misleading students into thinking all monoatomic ions have identical radii.
  • Overlooking charge alignment: Option C arranges identical charges side-by-side. In an ionic crystal, like charges adjacent to one another would cause immediate lattice instability due to mutual electrostatic repulsion.

Topics

Physical Chemistry · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.