The charges of Ca²⁺ ions and O^2- ions in CaO is higher than that of K⁺ ions and F^– ions in KF. The higher the ionic charge, the higher the electrostatic force of attraction between the oppositely charged ions, so more energy is required to overcome the electrostatic force in CaO than in KF.

Hence, CaO would have a higher melting point than KF.