Group 7 elements are called the Halogens. As you go up group 7 (decreasing atomic number), the elements become more reactive. For example, fluorine is the most reactive and astatine is the least reactive.
A more reactive halogen will displace a less reactive halogen, e.g. chlorine will displace bromine:
By reacting a halogen solution with a potassium halide solution and making observations, the order of their reactivity can be deduced:
|Potassium chloride, KCl(aq)||Potassium bromide, KBr(aq)||Potassium iodide, KI(aq)
|Chlorine, Cl2(aq)||No change||Colourless to orange||Colourless to brown
|Bromine, Br2(aq)||No change||No change||Colourless to brown
|Iodine, I2(aq)||No change||No change||No change
From the above results, chlorine displaces both bromine and iodine, and bromine displaces iodine. Therefore the order of reactivity is: chlorine is more reactive than bromine, which in turn is more reactive than iodine.