# 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

> 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.

## 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

- [crystal structure](https://www.gradenine.co.uk/glossary/crystal-structure)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-caesium-chloride-cscl-adopts-dbcd3767) · Published by Druglandscape Ltd.