The ionic radius of the cesium ion is 1.88 Å and that of the chloride ion is 1.67 Å. 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 (Cl-) is expected to prefer a cubic hole.
The images below depict the structure of crystalline CsCl. The red spheres represent the cesium ions and the green spheres represent the chloride ions.
Examine the images and note how the chloride ions form a simple cube with a cesium ion situated at the center. 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. The insertion of chloride ions into the cubic holes causes the structure to expand so that the cesium ions are not in contact with each other. Or one can describe the structure as a cubic array of chloride ions with cesium ions inserted into the cubic holes to force the chloride ions apart. The unit cell depicted below shows chloride ions surrounding a cesium ion. Both Cs+ and Cl- exist in a cubic coordination enviroment, thus CsCl has (8,8)-coordination.
Also note that the cubic arrangement of cesium ions (or chloride ions) does not constitute a closest-packed structure. Review the closest-packed structures (hcp and ccp) and note that neither closest-packed structure contains cubic holes.
The dimensions of the CsCl unit cell are a = b = c = 4.123 Å.
- How many cesium ions lie inside the unit cell?
- How many chloride ions like inside the unit cell? The structure as a whole is electrically neutral and thus the unit cell must be electrically neutral. Therefore Cs+ and Cl- ions must be present in a 1:1 ratio.
- What is the volume of the CsCl unit cell (in cm3)?
- What is the density of CsCl (in g cm-3)?
|Unit Cell||Portions of Atoms lying inside the Unit Cell|
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