Reactivity of halogens

The non-metal elements in Group 7 - known as the halogens - get less reactive as you go down the group. This is the opposite trend to that seen in the alkali metals in Group 1 of the periodic table.

Fluorine is the most reactive element of all in Group 7.

You can see the trend in reactivity if you react the halogens with iron wool.

HalogenReaction with iron wool
FluorineReacts with almost anything instantly. Very few scientists handle fluorine because it is so dangerous.
ChlorineReacts with heated iron wool very quickly.
BromineHas to be warmed and the iron wool heated. The reaction is faster.
IodineHas to be heated strongly and so does the iron wool. The reaction is slow.