What happens to the reactivity of nonmetals within a group from the top of the group to the bottom?
Reactivity decreases down a column of the nonmetals
Since these atoms react generally by acquiring electrons (through ionic or covalent bonding) if the atom has less energy to "give" when an electron is added (the electron affinity, as it is known), the reactions are less vigorous.
This decrease in electron affinity occurs because the valence shell is further from the nucleus as you proceed down the column, and so, the added electron is reduced in energy only slightly in dropping into this shell.
For instance, fluorine may just be the most reactive of all the elements, while iodine is only mildly corrosive, and has long been used (although perhaps not wisely) in an antiseptic solution and applied to open cuts.