Why is chlorine more reactive than iodine?

Chlorine has fewer shells than iodine - chlorine has 2 and iodine has 4. We know this by referring to the periodic table. This means its outer electrons are closer to the nucleus.Because they are closer - outer electrons feel more attraction towards the nuclear charge. Chlorine's nucleus can attract outer electrons more easily as there is less shielding of the nucleus.Chorine can therefore gain an electron more easily which makes it more reactive.