Halogens act as oxidising agents in redox reactions because they gain electrons and are themselves reduced.

The oxidising ability of halogens decreases as we move down group 7.

This is because:

1. As you go down the group, atomic radius and shielding increase (which outweighs the increased nuclear charge).
2. This means there is a weaker electrostatic attraction between the nucleus and the electron it is trying to gain.
3. This means their tendency to gain electrons and form negative ions decreases.

The thermal stability of hydrogen halides decreases as we move down group 7.

This is due to the increasing radius of the halogen atom which results in a longer and weaker hydrogen-halogen which requires less energy to break.

X₂ + H₂ ➔ 2HX

Both chlorine and bromine react with hydrogen to form hydrogen halides.

Chlorine reacts explosively in sunlight, while bromine reacts slowly on heating.

Aqueous bromide ions react with aqueous chlorine to produce a colour change to yellow due to the formation of bromine.

With iodine, there is no reaction observed because iodine is less reactive than bromine.

2I^-_(aq) + Br_2(aq) ➔ I_2(aq) + 2Br^-_(aq)

The reactivity of halogens decreases as we move down group 7. 

This is because:

1. As you go down the group, atomic radius and shielding increase (which outweighs the increased nuclear charge).
2. This means there is a weaker electrostatic attraction between the nucleus and the electron it is trying to gain.
3. This means more energy is required for a reaction to occur.

A reaction in which a more reactive halogen displaces a less reactive halogen from a solution of its salt.

Bromine forms a yellow solution.

Iodine forms an orange/brown solution.

2I^-_(aq) + Cl_2(aq) ➔ I_2(aq) + 2Cl^-_(aq)

Flourine