Why does reactivity increase going down a group on the periodic table?

Explanation:

Reactivity increases down a group because the elements are becoming bigger and the nuclear pull on the valent electron is fast becoming weaker. The availability of the valence electrons for reaction determines the reactivity of a chemical specie.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.