Why are Group 1 metals more reactive as you go down but Halogens are the opposite?Watch this thread
Hi its to do with the position of the electrons on the shells. As you go down group 1 the electrons become further away from the nucleus so there is less attraction , meaning electrons are lost more easily ; its the other way round for halogens.
The decrease in reactivity of the halogens is more to do with the decrease in electronegativity down the group. Fluorine is the most electronegative element so it can rip electrons of almost anything making it very reactive