Why basicity decreases down the group ?

• Basicity order: NH₃ PH₃ AsH₃ SbH₃≥ BiH₃

Due to the presence of lone pairs of electron, the hydrides of these elements are basic (Lewis bases) in nature. The basicity decreases with the size of the central atom 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.

Also, we can say basicity decreases down the group as the electronegativity decreases down the group. Electronegativity of N is highest. Therefore, the lone pairs will be more concentrated towards nitrogen and hence more repulsion between bond pairs ocuurs. Therefore bond angle is the highest. After nitrogen, the concentration of the lone pairs decreases with less repulsion between bond pairs with decrease in electronegativity down the group and so the basicity.

Basicity decreases as NH3 > PH3 > AsH3 > SbH3 > BiH3.