Question

Why does basicity decrease down a column?

Answer 1

due to diffusion of electrons over large volume i.e. down the group, as the size of the elements increases the electron density on element decreases.

Or we can say,

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

Answer:

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: ... And thereby the metallic character of the atom increases and thence the basicity decreases.