Question

why does basicity decrease down the group?

Answer 1

On going down the atomic radius increases so ionisation energy decrease, this means that tendence of losing electron increases which implies increase in metallic character ( as electron affinity also decreases ).

Answer 2

The basicity decreases when one goes down the group in a periodic table with the elements, because of the increasing size of the atoms on going down the group.

Explanation:

The elements placed down the group in the periodic table have their atomic size in an ascending order with relevance to the other, thereby when the atom size increases, atomic radius increases, and when the atomic radius increases, electron density of the atom decreases, so, when the electron density decreases, ionization energy also decreases along with electron affinity. And thereby the metallic character of the atom increases and thence the basicity decreases.