The distance between ${\mathrm{Na}}^{+}\text{and }{\mathrm{Cl}}^{-}$ions in NaCl having a density of 3.165 g/cc is

$\begin{array}{l}\mathrm{d}=\frac{\mathrm{Z}\times \mathrm{M}}{{\mathrm{a}}^3\times {\mathrm{N}}_{\mathrm{A}}}\\ {\mathrm{a}}^3=\frac{\mathrm{Z}\times \mathrm{M}}{\mathrm{d}\times {\mathrm{N}}_{\mathrm{A}}}=\frac{4\times 58.5}{3.165\times 6.023\times {10}^{23}}\\ {\mathrm{a}}^3=122.77\times {10}^{-24}{\mathrm{cm}}^3\\ \mathrm{a}=4.97\times {10}^{-8}\mathrm{cm}\text{ or }497\mathrm{pm}\end{array}$

Distance between ${\mathrm{Na}}^{+}\text{and }{\mathrm{Cl}}^{-}=\frac{a}{2}=\frac{497}{2}=248.5\mathrm{pm}$