Question

In the rock salt structure, the number of formula units per unit cell is equal to

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

The correct option is D 4

In Nacl (rock salt): Number of $N{a}^{+}$ ions = 12 (at edge centers) $\times \frac{1}{4}+1$ (at body center) $\times$ 1 = 4.
Number of $C{l}^{-}$ ions = 8 (at corners) $\times \frac{1}{8}+6$ (at face center) $\times \frac{1}{2}=4$. Thus 4 formula units per unit cell.