Substances made of small molecules are usually gases or liquids. Examples are methane, CH4, and oxygen, O2. They have relatively low melting points and boiling points.
The atoms inside each molecule are joined by strong covalent bonds, but between the molecules there are only weak forces. These are called intermolecular forces. When a small-molecule substance melts or boils, it is the intermolecular forces that are overcome, not the covalent bonds. Only a small amount of energy is needed to overcome weak forces, so the melting point and boiling point are low. The molecules stay whole, so liquid methane boils to give gaseous methane molecules, not separate atoms.
The intermolecular forces increase with the size of the molecules. This means larger molecules have stronger forces between them, so they have higher melting points and boiling points than smaller molecules.
Small-molecule substances do not conduct electricity. The molecules do not have an overall electric charge, so there are no charged particles free to move and carry current.
