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

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

The Solid State

Solution:

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 comers) $\times \frac{1}{8}$
$+6$ (at face centre) $\times \frac{1}{2} = 4$ .
Thus $4$ formula units per unit cell.