Covalent compounds have lower melting and boiling points due to presence of weak inter-molecular forces by which molecules are weakly associated to each other. Hence, melting and boiling of covalent compounds occurs at low temperatures.
Usually, Covalents compounds have following properties:
They are brittle solids
They do not conduct electricity
They do not have rigid crystal structure.
They exist in all states: solids, liquids and gases.