Some elements are more reactive than others. A reactive metal will displace a less reactive one from a compound for example, if you put an iron nail in a solution of copper sulphate, the nail will gradually become coated with copper.
Iron is more reactive than copper.
You can use displacement reactions to work out the order of reactivity of the halogens. The word equations below show two reactions which can help you to work out which halogens are most reactive:
Potassium Bromide + Chlorine --- ________________
Potassium Chloride + Bromine --- ________________