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)`?

Explanation

In MgO, both the ions carry two units of charge `(Mg^(2+), O^(2-))` whereas in NaF, each ion carries only one unit charge
`(Na^(+)F^(-))`,Therefore, there are stronger electrostatic forces of attraction in MgO and, hence, more energy is required
to overcome these forces. Thus, its melting point is high.