Answer
Covalent bonds are highly directional, and their fundamental units are individual molecules. But ionic bonds have are non-directional and hold together an entire array of ions. In covalent compounds, the interaction between molecules is weaker, so they have lower boiling and melting points than ionic compounds.
Work Step by Step
The Lewis model explains that two covalently bonded atoms share one or more electron pairs between them, which causes attraction between them, and so the fundamental units of covalent compounds are individual molecules which have strong intramolecular forces, but weaker intermolecular forces than ionic compounds. Since the processes of boiling and melting need to overcome just the intermolecular forces, the covalent compounds have a lower melting and boiling point than ionic compounds.
