Answer: 219 pm.
- A 90 pm
- B 154 pm
- C 219 pm
- D 370 pm
Correct answer: C. 219 pm
Explanation: NaCl structure requires radius ratio r(cation)/r(anion) = 0.414 to 0.732. Minimum: r(M⁺)/r(X⁻) = 0.414; r(X⁻) = 90/0.414 ≈ 217 pm ≈ 219 pm.
The three cubic unit cells: Simple Cubic has atoms only at corners; Body-Centred adds one atom at the centre; Face-Centred adds one atom at the centre of each of the 6 faces, giving the highest packing efficiency.
Concept context
Explore the ordered world of crystalline solids: unit cells, packing, defects, and how structure determines electrical and magnetic properties.