Which compound would be expected to have the highest lattice energy?

Explanation

Magnesium oxide is built from doubly charged Mg2+ and O2- ions that are also small, so both factors in the electrostatic expression work in its favour and its lattice energy is roughly four times that of sodium chloride. Among the singly charged compounds listed, lattice energy falls as the ions get larger, so NaCl exceeds KCl, which exceeds KBr. Melting points follow the same order.