Shielding

Although the nucleus contains more protons further down the group, the greater distance and shielding mean that its attraction for the outer electron is weaker overall.

Although nuclear charge increases down a group, the increased distance and shielding dominate in the trends considered here. The result depends on whether reaction requires an electron to be lost or gained.

An incoming electron is attracted to the positive nucleus, but the outer shell is further away and there is more shielding down the group. Gaining an electron becomes less easy. Thus, given the trend, bromine is predicted to be more reactive than iodine because bromine is higher in Group 7.