The ions of NaF and MgO all have the same number of electrons and the internuclear distance are about the same (235pm and 215pm).Why then are the melting points of NaF and MgO so different (992 degree C and 2642degree celcius)?

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Solution

The crystals of NaF and MgO are formed by Na+ and F- in NaF and Mg2+ and O2–ions in MgO respectively arranged in cubic closed structures. There are strong electrovalent bond forces (strong coulombic forces, attraction between ions) between Na+and F– in NaF and Mg2+ and O2– in MgO. But the magnitude of these coulombic forces of attraction is much higher in MgO as compared to that in NaF. (Mg2+ is divalent while Na+is monovalent, similarly O2– and F–). The electrostatic forces of attraction between Mg2+and O2– is almost 4 times as compared to Na+ and F– ions. Melting point of ionic solids is almost the index of inter-ionic attraction in crystal lattice as lot of energy is required to break these forces/overcome these forces before the substance melts.

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