How does the reactivity of group 7 elements (the halogens) change down the group and why? (A Level)

As you move down group 7 from fluorine to iodine, the reactivity of the elements decreases. The halogens are non-metals and when non-metals react with metals, they GAIN the electrons that the metals lose. As you move down group 7, the atomic radius get bigger (more protons, more electrons, more shells) and so the negatively charged electrons in the outer shell move further away from the positively charged nucleus. This means that the attraction between the two is weaker. The weaker attraction makes it more difficult for the atom to pull an electron from a metal, resulting in a weaker reaction.