Lattice Energy ($U$) is the energy released when one mole of an ionic crystalline compound is formed from gaseous ions. According to Coulomb's Law: $U\propto \frac{q_1{q}_2}{r^0}$ Where $q_1,q_2$ are the charges and $r_0$ is the distance between ions.

• In MgO, ions are $M{g}^{2+}$ and $O^{2-}$. The product of charges is $4$.
• In NaCl, ions are $N{a}^{+}$ and $C{l}^{-}$. The product of charges is $1$.
• In KBr, ions are $K^{+}$ and $B{r}^{-}$. The product of charges is $1$.
• In CaCl₂, ions are $C{a}^{2+}$ and $C{l}^{-}$. Higher charge on $Ca$, but distance is larger. Since $MgO$ has high charges and small ionic radii, it has the highest lattice energy.