Fluorine, chlorine, bromine and iodine are all elements in Group 7 of the periodic table. A student states: 'Fluorine is the most reactive element in Group 7 because it has the smallest atom.' Explain why the student's statement is correct, and describe how the reactivity of the Group 7 elements changes going down the group.

Fluorine is the most reactive element in Group 7 because it has the fewest electron shells. The outer shell, where the new electron is gained, is therefore closer to the nucleus in fluorine. This gives fluorine a stronger attraction for an incoming electron, so it gains an electron more easily.

Reactivity decreases going down the group. As we move to chlorine, bromine and iodine, the atoms have more electron shells; the outer shell is further from the nucleus, so an incoming electron is attracted less strongly. Consequently, the reactivity of the Group 7 elements falls down the group.