Question

Which is mismatched for NaCl crystal?

• A. \( \frac{r^+}{r^-} = 0.414 \text{ to } 0.732 \)
• B. Coordination number = \(6:6\)
• C. Edge of unit cell = \( r^+ + r^- \)
• D. Crystal structure = fcc

Hint:

NaCl: \(a = 2(r^+ + r^-)\), not just \(r^+ + r^-\).

Answer 1

The Correct Option is C

Concept: NaCl structure is face-centered cubic.

Step 1: Correct relations.

• Coordination number = \(6:6\)
• Radius ratio = \(0.414\) to \(0.732\)
• Structure = fcc

Step 2: Edge length relation.
Correct formula: \[ a = 2(r^+ + r^-) \] Conclusion: Option (C) is incorrect.