Reactivity of elements follow a trend in such a manner that elements in the same group have similar chemical properties since they react in similar ways during chemical reaction. This similarity is as a result of the same number of valence electrons they have in their respective outer shell. This particular similarity is what classifies them into groups.
In this question, Chlorine is in the same group with Iodine and they're referred to as "halogens" and all halogen have similar chemical reactions. Example, they seek one valence electron to be stable. Easy formation of salts with metals etc
