Which of the following statements about the $NaCl$ crystal structure are correct?

AEach $Na X^{+}$ has 12 next-nearest $Na X^{+}$ neighbours.
BThe 8 tetrahedral voids of the $Cl X^{-}$ lattice in each cell are all occupied by $Na X^{+}$ .
CWhen cations and anions touch along the edge, the cell edge is $a = 2 (r_{+} + r_{-})$ .
DThe $Na X^{+}$ ions occupy all the octahedral voids of the ccp $Cl X^{-}$ lattice.

Answer: A, C, D

The

Na X^{+}

ions form an fcc array themselves, so each has 12 like neighbours (true). The edge contains

r_{-} + 2 r_{+} + r_{-}

... i.e. one cation diameter and two anion radii, so

a = 2 (r_{+} + r_{-})

(true). The 4 octahedral voids hold 4

Na X^{+}

(true). The tetrahedral voids are empty in rock salt (false).