The halogens are members of Group 17 on the periodic table. The halogens all exist as diatomic molecules in nature i.e. two atoms joined together by Van der Waals dispersion forces. It includes fluorine, chlorine, bromine, iodine and astatine. The heavier halogens have more electrons in their valance shells. When electrons moves towards the nucleus of an atom at one time, there may be more electrons on one side of a molecule, creating a temporary negative charge on that side and a temporary positive charge on the other side -- dipole. The temporary negative and positive poles of different molecules attract each other, and the sum of the temporary forces results in a weak intermolecular force. As a results, Van der Waals forces becomes stronger between the molecules of heavier halogens It takes more heat to overcome these stronger forces, meaning that the boiling points are higher for heavier halogens. Van der Waals dispersion forces are the weakest intermolecular forces, so the boiling points of the halogens as a group are generally low.