Answer:
Step-by-step explanation:
Reactivity of halogens basically decreases. Imagine this halogens are these people who need love, they need to attract metals who are hot.
The number of shells increases as you go down the group, so they become fatter. Fluorine is the hottest , chlorine is the second hottest, the rest are fat-fattest. The plus size elements (Bromine, Iodine) find it easier to lose an electron, because they are "insecure", the attraction between protons and valence electrons are not high, because they are so far away. Metals need their partners do be stiff so that they can give away their electrons. Halogens are not metals, they need to attract them, so it is ideal here to not lose electrons.
Hope that explanation helps, phew!