Describe the trend in boiling point of the halogens.

The boiling point of the halogens increases as you go down the group. The halogens exist as diatomic molecules, with both atoms sharing an electron to completely fill the outer shell. The increase in boiling (and melting point) can be attributed to the increase in intermolecular forces (van der Waals). The number of electrons increases in each element going down the group, this leads to an increase in temporary dipoles which can be set up. (side note, van der Waals forces: electrons move randomly around the nucleus of an atom, at any one time there may be more electrons on one side of a molecule creating a temporary negative charge on that side and a positive charge on the other - an instantaneous dipole. This is what holds the halogens together) In addition, the size of the molecules increases going down the group - this gives more 'space' for the electrons to sit in, this results in a reduced repulsion i.e. the electrons repel each other less. However, this is a side factor and is only particularly relevant for fluorine. This means that it is required to supply more heat in order to overcome these forces.