Why do melting and boiling points of halogens increase on moving down the group?
Answer»
The atomic size of halogens increases on moving down the group and hence the strength of van der Waals forces between the molecules also increases.
In the lighter elements, F and Cl, these forces are weak, hence they are gases at room temperature and possess low melting and boiling points.
In the heavier elements of the group, the van der Waals forces become stronger. Thus, bromine is a liquid and iodine is a solid and they possess higher melting and boiling points.