Why does the reactivity of group 7 elements decrease down the periodic table?

As you go down the group, the outer electrons are further from the nucleus, this leads to a reduced attraction between the negative outer electrons and positive nucleus. Also, as there are more electrons, the inner shells actually repel the outer most electrons as they have a common charge. Due to the weaker attraction of electrons to the positive nucleus, it is harder for the atom to gain one electron to complete their outer shell so they become less reactive as more energy is required to complete the electron shell