The lattice enthalpies of calcium oxide and magnesium oxide are different. Comment on this difference.

Differences in size of an ionic lattice are directly linked to differences in how exothermic the lattice enthalpy of a salt is. 

Mg²⁺ has a smaller radius than Ca²⁺  and thus a higher positive charge density.

This results in a stronger affinity for the negative O^2- ions.