Forces between molecules
For example, covalently-bonded substances like hydrogen have weak intermolecular forces. As such, hydrogen has a low melting point and a low boiling point and is a gas at room temperature. Covalently-bonded molecules are usually liquids or gases at room temperature due to the weak intermolecular forces between each molecule. If the intermolecular forces are stronger between the molecules, more energy (heat) is requried to overcome the forces and melt / boil it.