How does reactivity vary within a group amongst the halogens

Question

How does reactivity vary within a group amongst the halogens?
(increase/ decrease?)

What is the trend amongst the halogens?
(Why does the reactivity vary this way?)

Answer 1

the general reactivity of halogens (to form ionic compounds) decreases down the group.
when those elements goes down the group, the number of filled electron shells increases. due to the screening effect, the attraction between the nucleus and the coming electron (donated by the cation, eg, Na donates an electron to Br, forming Na+ and Br-) is reduced down the group.
since electrons are less easy to be accepted by the halogen atoms, the tendency for the atoms to form ions decreases down the group. therefore the reactivity decreases down the group.