Question

Why do melting and boiling points of halogens increase on moving down the group?

Answer 1

• 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.