The ionic radius of the cesium ion is 1.88 angstroms and that of the chloride ion is 1.67 angstroms. In this case the cation is the larger ion, and the ratio of radii for the anion and cation is r-/r+ = 1.67/1.88 = 0.888.
With a radius ratio of 0.888, the smaller ion is expected to prefer a cubic hole.
The images below depict the structure of CsCl. The green spheres represent the chloride ions and the red spheres represent the cesium ions.
Examine the images and take note of the following points:
1. The chloride ions lie in a simple cubic arrangement. Owing to the symmetry of the structure, it does not matter whether one regards the chloride ions as adopting a cubic structure with cesium ions inserted into cubic holes or cesium ions adopting a cubic structure with chloride ions inserted into cubic holes.
2. The cesium and chloride ions are comparable in size.
3. The insertion of chloride ions into the octahedral holes causes the structure to expand so that the cesium ions are not in contact with each other. (The unit cell actually shows chloride ions surrounding a cesium ion. Envision several adjoining unit cells, which produce cesium ions surrounding a chloride ion.)
4. All of the octahedral holes are occupied by cesium ions. The ionic solid is electrically neutral and the unit cell itself must also be electrically neutral. Examine the unit cell and verify that it contains equal numbers of cesium and chloride ions.
5. CsCl has (8,8)-coordination.
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