Atoms in molecules are held together by strong covalent bonds which act only between the atoms within the molecules. Simple molecules have little attraction for each other with relatively low melting points and boiling points.
The forces of attraction between molecules, 'intermolecular forces', are weak. They overcome when molecular substances melt or boil. Those with the smallest molecules, H2, Cl2 and CH4, have the weakest intermolecular forces and are gases at room temperature. Larger molecules have stronger attractions and so may be liquids at room temperature, like Br2 and C6H14, or solids with low melting points.
Molecules have no overall charge and cannot carry an electric current.
Covalent bonds are strong and difficult to break; intermolecular forces are much weaker and more easily overcome.
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