More ionic is a compound, stronger would be the ionic bond and morewould be the lattice enthalpy. In MgO, the ionic bond is stronger than in NaClbecause of more charge on the ionicspecies in the former (Mg+2 and O2- as compared to Na+ and Cl-). Because of this the ionic interactions are stronger in MgO than in NaCl.So MgO is more ionic than NaCl