why MgO has a lower mpt

The melting point of MgO is 2852°C.
(http://en.wikipedia.org/wiki/Magnesium_oxide)

The melting point of Na2O is 1132°C.
(http://en.wikipedia.org/wiki/Sodium_oxide)

The melting point of MgO is higher than that of Na2O.

Reason :
The attraction between Mg^2+ and O^2- ions is the electrostatic attractionbetween a doubly-charged cation and a doubly-charged anion, while theattraction between Na^+ and O^2- ions is the electrostatic attraction between asingly-charged cation and a doubly-charged anion. Besides, Mg^2+ has a smaller size than Na^+. Therefore, the attraction between Mg^2+ andO^2- ions is stronger than that between Na^+ and O^2- ions.