WJEC A-Level Chemistry AS Unit 1, June 2025: Question 2

✅ Acceptable Answers [1 Mark]

Any one of the following statements is awarded 1 mark:

• Na⁺ ion is smaller (than Cs⁺ ion)
• Cs⁺ ion is larger (than Na⁺ ion)
• Na⁺ and Cs⁺ have different ionic radii / sizes

💡 Key Knowledge

• Coordination Number: The number of nearest neighbouring oppositely charged ions surrounding a given ion in an ionic crystal.
  • In NaCl, coordination number is 6:6 (each Na⁺ is surrounded octahedrally by 6 Cl⁻ ions, and vice versa).
  • In CsCl, coordination number is 8:8 (each Cs⁺ is surrounded cubically by 8 Cl⁻ ions, and vice versa).
• Radius Ratio Effect: Because the Cs⁺ ion has more electron shells than Na⁺, it has a significantly larger ionic radius. More chloride ions can pack around a larger Cs⁺ ion without like charges repelling one another.

🧠 Exam Technique & Examiner Commentary

• Refer to the ions, not atoms: Always specify that it is the Na⁺ ion or Cs⁺ ion, or refer to ionic radius rather than atomic radius.
• Comparative language: A simple comparative statement like "Na⁺ is smaller than Cs⁺" is the fastest, safest way to secure the mark.
• Don't overcomplicate: This is a 1-mark question. You do not need to calculate radius ratios or explain octahedral vs cubic packing unless asked; a direct statement of ion size is all that is required.

❌ Common Errors & Misconceptions

• Talking about charge: Stating that they have different charges. Both Na⁺ and Cs⁺ have the same charge (+1), so charge difference cannot explain the difference in coordination number.
• Referring to atoms instead of ions: Writing "sodium is smaller than caesium" instead of referring to the ions (Na⁺ and Cs⁺).
• Confusing the anions: Mentioning chloride ion differences. The anion (Cl⁻) is identical in both compounds; only the cation changes!
• Electronegativity explanations: Vaguely talking about electronegativity differences or covalent character rather than physical packing geometry and ionic radii.