What do periodic trends of reactivity occur with the halogens?
Hello and thanks for the question regarding the reactivity of the halogens.
As you go down the group (fluorine down through astatine), the reactivity lessens. Fluorine is the most active halogen and iodine is the least reactive halogen. Astatine is rare and normally not used in any reactions that we write or use.
Fluorine is also the most electronegative element on the periodic table, thus it has been described as the most active nonmetal.
Hope this helps.