Chemistry - Thermochemistry Question with Solution | TestHub

$$\text{ Lattice energy }=-\frac{q_1{q}_2}{r^2}$$ where $q_1$ and $q_2$ are charges on ions and $r$ is the distance between them. Since interionic distances in $\mathrm{CaO}$ and $\mathrm{NaCl}$ are similar, (larger cation has smaller anion and vice versa) therefore, $r$ is almost the same. Therefore, lattice energy depends only on charge. Since the magnitude of charge on ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}$ions is unity and that on ${\mathrm{Ca}}^{2+}$ and ${\mathrm{O}}^{2-}$ ions is 2 each, therefore, the lattice energy of $\mathrm{CaO}$ is four times the lattice energy of $\mathrm{NaCl}$, i.e., $4U$.