Question

Why melting point of MgO is much higher than that of BaO

Answer 1

Because of difference in the lattice energy!

They all have the same FCC lattice structure, but the lattice energy of MgO is roughly 4 time of that NaCl and KCl because it is proportional to Z+•Z-.

Furthermore, Mg2+ and O2- both have a electron configuration of Ne, while Ba2+ has a Xe’s one. So the distance between Ba2+ and O2- is larger than that between Mg2+ and O2-. Since lattice energy is disproportional to this distance, that could be why the lattice energy of MgO is larger than that of BaO.

Lattice energy higher, melting point point higher!