The ionic radius of the sodium ion is 1.16 angstroms and that of the chloride ion is 1.67 angstroms. The ratio of radii for the cation and anion is thus r+/r- = 1.16/1.67 = 0.695.
With a radius ratio of 0.695, the cubic holes are too large (rhole/r = 0.732) to be suitable. The sodium ions will prefer to occupy octahedral holes in a closest-packed structure. As it happens, the chloride ions in NaCl pack in a cubic closest-packed structure.
The images below depict the structure of NaCl. The green spheres represent the chloride ions and the red spheres represent the sodium ions.
Examine the images and take note of the following points:
1. The chloride ions lie in a cubic closest-packed arrangement.
2. The sodium ions are smaller than the chloride ions.
3. The insertion of sodium ions into the octahedral holes causes the structure to expand so that the chloride ions are not in contact with each other.
4. All of the octahedral holes are occupied by sodium ions. The ionic solid is electrically neutral and the unit cell itself must also be electrically neutral. Because the chloride ions adopt a ccp structure, there are four chloride ions in the unit cell. Consequently there must also be four sodium ions in the unit cell. Examine the unit cell and verify this fact.
5. NaCl has (6,6)-coordination.
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