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Which of the following statements about the NaCl crystal (a = edge length of unit cell) is incorrect?

1. Nearest cation-cation distance is a / sqrt(2)
2. Na+ ions occupy the body centre and edge centres of the FCC unit cell formed by Cl- ions
3. Each Na+ ion is closely surrounded by 6 other Na+ ions
4. Nearest cation-anion distance is a / 2

Correct answer: Each Na+ ion is closely surrounded by 6 other Na+ ions

Solution

In rock-salt NaCl: Cl- ions form an FCC lattice; Na+ ions sit in octahedral holes (body centre and all edge centres). Nearest Na-Cl distance = a/2 (statement D: correct). Nearest Na-Na distance = a/sqrt(2) (face diagonal / 2 scaled by root 2; statement A: correct). Na+ at body centre and edge centres of the Cl- FCC (statement B: correct). However, each Na+ is surrounded by 6 Cl- ions (coordination number 6 with OPPOSITE ion). The nearest same-type (Na-Na) neighbours are 12 ions at distance a/sqrt(2), not 6. Statement C claiming 6 nearest Na+ neighbours is INCORRECT.

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