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. Produces iron(III) fluoride.
ChlorineReacts with heated iron wool very quickly, although not as quickly as fluorine does. Produces iron(III) chloride.
BromineHas to be warmed and the iron wool heated. The reaction is faster than that of iodine but slower than that of chlorine. Produces iron(III) bromide.
IodineHas to be heated strongly and so does the iron wool. The reaction is slow. Produces iron(III) iodide.
Diagram showing the reactivity of chlorine with iron wool, and how it produces iron(III) chloride.Chlorine reacts quickly, but not as quickly as fluorine

Iron + chlorine → iron(III) chloride

2Fe + 3Cl2 → 2FeCl3

Diagram showing the reactivity of bromine with iron wool, and how it produces iron(III) bromide.Bromine has a less vigorous reaction

Iron + bromine → iron(III) bromide

2Fe + 3Br2 → 2FeBr3

Diagram showing the reactivity of iodine with iron wool. Produces iron(III) iodide.Iodine has the slowest reaction

Iron + iodine → iron(III) iodide

2Fe + 3I2 → 2FeI3