Explain why caesium chloride (CsCl) adopts a different crystal structure from sodium chloride (NaCl), even though both are ionic compounds with a 1:1 cation-to-anion ratio.

IB DP Chemistry Higher Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Caesium chloride crystallises in a body-centred cubic arrangement in which each Cs⁺ ion is surrounded by eight Cl⁻ ions, whereas sodium chloride crystallises in a face-centred cubic arrangement in which each Na⁺ ion is surrounded by six Cl⁻ ions.

Model answer (4 marks)

The crystal structure adopted by an ionic compound depends on the radius ratio of the cation to the anion. Cs⁺ is much larger than Na⁺, giving CsCl a larger radius ratio. A larger radius ratio allows the cation to be surrounded by more anions without causing anion–anion repulsion, so CsCl adopts a body‑centred cubic structure with coordination number 8. In contrast, the smaller Na⁺ has a smaller radius ratio; it can only accommodate six Cl⁻ neighbours before anion–anion repulsion destabilises the lattice, so NaCl adopts the face‑centred cubic rock‑salt structure with coordination number 6.

Examiner tips

  • Mention the radius ratio rule first; link size to coordination number; explain how larger ratio leads to 8‑fold coordination; contrast with 6‑fold for NaCl.

Common mistakes

  • Confusing coordination number with lattice type; not stating the radius ratio rule; ignoring the effect of cation size on anion–anion repulsion.

Mark scheme (4 marks)

  1. The structure adopted depends on the ratio of the radius of the cation to the radius of the anion (the radius ratio).
  2. Cs⁺ is a much larger cation than Na⁺, giving CsCl a larger radius ratio than NaCl.
  3. A larger radius ratio means the cation can accommodate more anions around it without anion–anion repulsion, allowing a coordination number of 8 in CsCl.
  4. In NaCl the smaller Na⁺ can only support six Cl⁻ neighbours (coordination number 6) before anion–anion repulsion destabilises the lattice, so the rock-salt structure is adopted instead.

Key terms in this question

crystal structure

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