Predict whether the lattice energy of magnesium oxide, MgO, is more or less exothermic than the lattice energy of magnesium sulfide, MgS. Justify your answer in terms of the sizes and the charges of the ions involved.

To start with, we can clearly see that the difference between MgO and MgS is the anions, we have O(2-) and S(2-). As we know (from looking at the periodic table) O(2-) has a smaller ionic radius, and thereby higher charge density than S(2-), as it has the same charge but packed into a smaller space. As a result, O(2-) will form stronger electrostatic attractions i.e. ionic bonds to Mg(2+) due to the smaller ionic radius and higher charge density, thereby MgO will have a more exothermic lattice enthalpy due to the stronger ionic bonds relative to MgS.