If you test different combinations of the halogens and their salts, you can work out a reactivity series for Group 7:
It doesn’t matter whether you use sodium salts or potassium salts – it works the same for both types.
The slideshow shows what happens when chlorine, bromine and iodine are added to various halogen salts:
Adding chlorine, bromine and iodine to halogen salts
Halogen displacement reactions are redox reactions because the halogens gain electrons and the halide ions lose electrons.
When we consider one of the displacement reactions, we can see which element is being oxidised and which is being reduced.
bromine + potassium iodide → iodine + potassium bromide
Br2 + 2KI → I2 + 2KBr
As an ionic equation (ignoring the ‘spectator’ potassium ions):
Br2 + 2I– → I2 + 2Br–
We can see that the bromine has gained electrons, so it has been reduced. The iodide ions have lost electrons, so they have been oxidised.
As we descend Group 7, the reactivity decreases. For stability, the atom needs to have a full outer shell. Group 7 elements need to gain 1 electron to have a full shell.
As a result, a negative ion is formed:
Cl + e– → Cl–
As we descend the group: