Question

Given `MgO`, `Li_2O`, `MgCl_2`, and `NaO`, why should `MgO` have the highest lattice enthalpy?

Answer 1

Because lattice enthalpy depends on two things: charge; and size.....

Explanation:

Depending on your definition, the lattice enthalpy is the energy released when equivalent quantities of (gaseous) cation and anion bind together to form an ionic lattice.

`Mg^(2+)` is the smallest cation, and also it is the only that is a dication. Oxide anion is also the smallest anion, and it is the only that is a dianion. On this basis, the ionic bond(s) formed between magnesium and oxide ions should be of the greatest magnitude.

You should search yourself for the ionic radii, in order to put some numbers on your argument.