How does the reactivity down group 1 and 7 differ and how is this related to the atomic radius of these elements?

Explanation:

Group 1 elements have a single electron (called the valence electron) in there outermost shell. Reactivity of group 1 elements (alkali metals) increases down a group; this is because as one moves down the group, the atomic radius increases thus causing the outermost electron/valence electron to be found further away from the nucleus, which makes it (the electron) to be easily lost due to weaker force of attraction between the single valence electron and the central nucleus.

However, in the case of group 7/halogen elements, as one moves down the group, the atomic radius also increases, but the reactivity reduces. This is because as one moves down the group and the atomic radius increases, the nucleus ability to attract an electron lowers, because of the increased number of electrons that shields against the pull of the nucleus (this is referred to as nuclear shielding). This is the reason fluorine is the most reactive halogen.