Explain why chlorine is more reactive than iodine.

Both Cl and I in Group 7. Both have 7 electrons in its outer shell and require 1 electron to obtain a full shell. Reactivity of these halogens correspond to its ability to gain an electron. Cl can gain an electron more easily than iodine - Cl more reactive. This is because Cl is higher up Group 7 than iodine (Cl: 2,8,7 electron configuration). Cl outer shell closer to nucleus (and less shielded) than iodine outer shell. Therefore Cl has greater attraction for an electron.