State and explain the trend in boiling points of chlorine, bromine, and iodine

Boiling point increases down the group, because of increasing strength of van der Waals (intermolecular) forces. More energy is required to break van der Waals (intermolecular) forces.Follow up questions could be:1) What are van der Waals forces and what determines their strength?2) Why do all halogens have similar reactivity?