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Which of the following statements are correct for the rock-salt (NaCl) crystal structure?

1. Tetrahedral voids are smaller than octahedral voids
2. Cations occupy octahedral voids and tetrahedral voids are empty
3. The radius ratio (r+/r-) is 0.732
4. The radius ratio (r+/r-) is 0.999

Correct answer: Tetrahedral voids are smaller than octahedral voids

Solution

In the rock-salt structure, Cl⁻ ions form an FCC lattice and Na⁺ ions occupy all octahedral voids; tetrahedral voids are empty. Tetrahedral voids are always smaller than octahedral voids in any close-packed structure. The radius ratio for rock-salt falls in the range 0.414 to 0.732 (not 0.732 itself, which is the upper limit for octahedral, and not 0.999). Statements A and B are both correct.

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