Number of ion pairs present in 1 unit cell of sodium chloride is

Body centre = 1 Na⁺ x 1 = 1 Na⁺

Edge centre = 12 Na⁺  x  $\frac{1}{4} = 3 {\mathrm{Na}}^{+}$

Corner = $8 {\mathrm{Cl}}^{-} \mathrm{x} \frac{1}{8} = 1 {\mathrm{Cl}}^{-}$

Face centre = $6 {\mathrm{Cl}}^{-} \mathrm{x} \frac{1}{2} = 3 {\mathrm{Cl}}^{-}$

NaCl unit cell contains 4 ion pairs.