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.

In a small atom usually found at the top of the group, the nuclear pull on the electrons in the shell is huge. As we go down and the number of successive shells increases, the pull by the nucleus reduces. This makes electrons available for chemical reactions.

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