look............

acidic strenght also depends on

DISTRIBUTION OF CHARGE ON THE MOLECULE........

(UNDERSTOOD).

 

larger the distribution .............higher the acidic strength............

 

now as we go down...........

d electrons increases...............

thus  electrons are transferred to d block............( if not filled)

 

and as we go down...........

no. of d orbitals arefound empty............

right.......( that is d pi - dpi bonding)

 

therefore 

acidic strength increases.............

 

thanku