Question

Why do ionic solids tend to have melting points hundreds of degrees higher than molecular solids?

Answer 1

Because of the different strength of the key bonds.

Explanation:

Ionic compounds contain non-directional ionic bonds, which are strong. The solids are giant crystalline lattice structures and so there are many strong ionic bonds to break when the solid melts. This takes a lot of energy which is reflected in the higher melting points.

On the whole molecular solids are held together by weaker intermolecular forces e.g. iodine and so these take less energy to break upon melting or subliming in the case of iodine and so have lower melting points. There are, of course, macromolecular structures like silicon dioxide which has a very high melting and point due to the many convalent bonds that have to break.