Simple molecular substances, such as water, methane, carbon dioxide and hydrogen chloride, are made of small molecules. The atoms inside each molecule are joined by strong covalent bonds. Between one molecule and the next there are only weak intermolecular forces.
When a simple molecular substance melts or boils, the molecules move apart from each other. Only the weak intermolecular forces are overcome. The strong covalent bonds inside the molecules stay intact. Little energy is needed, so these substances have low melting points and boiling points, and many are gases or liquids at room temperature.
Simple molecular substances are poor conductors of electricity. The molecules have no overall charge, and there are no free ions or delocalised electrons to carry a current.
