why melting point of MgO>NaCl ...............?????????????

Answer :

Metals have a tendency to lose electrons from their outer shell in order to achieve a full (or empty!) outer shell. This leaves a metal ion with a positive charge. In solid metals, the outer electrons are delocalised throughout a lattice of metal ions, holding them together by the electrostatic attraction between the negative electrons and the positive ions. Mg has two electrons in its outer shell. Na has only one. So in Mg, there are twice as many delocalised electrons and the ions have a 2+ charge meaning that the attractive forces holding the lattice in place are much greater than in sodium. To melt the solid you have to overcome these forces using heat so you need more heat (and thus a higher temperature) to overcome the attractive forces in Mg.

Thanks