Why do molecular compounds have low melting points and low boiling points relative to ionic substances? ?

Because molecular bonds are not as strong as ionic bonds, they require less energy (heat) to break apart. This translate to less heat being required to move from one state to the next (melting, boiling, freezing, condensing) and thus a lower melting and boiling point.

Deposition is the transition of matter going from gas to solid without passing through the liquid phase.a.) gas—-> solid