Question

Which of the compounds would you expect to have the highest melting point NaCl, H2Se, O2, H2O, H2S, CO2, MgO?

Answer 1

`"MgO"` has the highest melting point, by a large margin.

Explanation:

Ionic compounds have higher melting points than covalent compounds with small molecules. So the answer must be `"NaCl"` or `"MgO"`.

Compare these compounds. Sodium chloride has +1 and -1 ion, magnesium oxide has +2 and -2. Also the charge centers are closer together for magnesium oxide because its oxide ion is much smaller than the chloride ion in sodium chloride. So the ions in `"MgO"` are more tightly bound, giving it a melting point of about 2800°C versus 801°C for `"NaCl"` and less still for the covalent compounds in the list.