Question

The ions of NaF and MgO have the same number of electrons and inter nuclear distances are about the same (235 pm and 215 pm). Why are then the melting points of NaF and MgO so different `(992^(@)C and 2642^(@)C)`?

Answer 1

Both the compounds are iso-electronic and inter nuclear distance are also about the same. However, the ions differ in their nature. In MgO, both the ions are divalent in nature `(Mg^(2+)and O^(-))` while in NaF, these are of monovalent nature. `(Na^(+) and F^(-))`. The electrostatic forces among the ions are divalent in nature `(Mg^(2+)and O^(2-))` while in NaF, these are of monovalent nature. `(Na^(+) and F^(-))`. The electrosstatic forces among the ions in the crystal lattice of MgO are stronger than in case of NaF. Therefore, the melting point of MgO is higher `(2642^(@)C)` as compared to NaF `(992^(@)C)`.