Question

How many molecules are there in the unit cell of sodium chloride.

• A. 2
• B. 4
• C. 6
• D. 8

Answer 1

Answer: (B)

4

Answer 2

In $NaCl$ (rock salt) : Number of $Na^{+}$ions $= 12$ (at edge centers) $\times \frac{1}{4} + 1$ (at body centre)$\times 1 = 4$. Number of $Cl^{-}$ions $= 8$ (at corners)$\times \frac{1}{8} + 6$ (at face centre) $\times \frac{1}{2} = 4$.

Thus 4 formula units per unit cell.