The melting point of MgO is higher than that of NaF because: Mg2+ is more positively charged than Na+ & O2- is more negatively charged than F-.
When the bond between magnesium and fluorine is being formed, the electrons must divide equally between the two fluorine atoms, giving each only one electron. When magnesium bonds with oxygen, the one oxygen atom gains both electrons, making it stronger. The greater the charge of ions that occurs during the formation of an ionic bond, the stronger the resulting bond.
Magnesium oxide (MgO) - An inorganic compound that occurs in nature as the mineral periclase. In aqueous media combines quickly with water to form magnesium hydroxide. Mag Oxide is used as an antacid and mild laxative and has many nonmedicinal uses.
Sodium fluoride is a colorless crystalline solid or white powder, or the solid dissolved in a liquid. It is soluble in water. It is noncombustible. It is corrosive to aluminum. It is used as an insecticide. It is also used to fluorinate water supplies, as a wood preservative, in cleaning compounds, manufacture of glass, and for many other uses.