Question

The crystalline structure of $NaCl$ is

• A. hexagonal close packing
• B. face-centred cubic
• C. square planar
• D. body-centred cubic

Answer 1

The correct option is B face-centred cubic

The crystalline structure of NaCl is face-centred cubic.
The total number of ions present in one unit cell of sodium chloride lattice is 8. It includes 4 $N{a}^{+}$ ions and 4 $C{l}^{-}$ ions.
There are 8 $C{l}^{-}$ ions at 8 corners of fcc unit cell (each one contributes one eigth to unit cell) and 6 $C{l}^{-}$ ions at 6 face centres (each one contributes one half to unit cell). Total number of $C{l}^{-}$ ions in the unit cell $=8\times \frac{1}{8}+6\times \frac{1}{2}=1+3=4$
$N{a}^{+}$ ions occupies octahedral voids. There are 4 octahedral voids in fcc unit cell. Thus, there are 4 $N{a}^{+}$ ions in fcc unit cell.