#"Normal melting point, magnesium oxide"# #=# #2852# #""^@C#.
#"Normal melting point, sodium chloride"# #=# #801# #""^@C#.
Neither #MgO#, nor #NaCl# are molecular species, and they are both extended arrays of anions and cations close-packed together in an ionic lattice. For magnesium oxide, we deal with #Mg^(2+)# and #O^(2-)# ions.......and thus there should be greater ATTRACTIVE interaction between these ions than between singly charged ions. Moreover, #Mg^(2+)# ions, and #O^(2-)# are SMALLER than #Na^(+)# ions, and #Cl^(-)#, and again this gives rise to greater (attractive) ionic interaction in the case of #MgO#.
Simple ideas with regard to electrostatics, for instance Coulomb's law, shows an inverse square law governing the force between charged particles, inversely proportional to the square of the distance between them.
For more details, you are going to have to check your notes with regard to #"lattice enthalpies"#; and the lattice enthalpy of the magnesium oxide salt is demonstrably greater in magnitude.
