Explain the trend in reactivity of the elements in Group 7.

The Halogens decrease in reactivity as you descend through the group. The atoms increase in size and Mr as you go down the group. Therefore their outer electrons are further away from the positive charge of the nucleus. There is a weakened electrostatic force of attraction between the nucleus and the outer electron shell and so it is harder for the atom to complete the outer shell with a new electron.