why is NH3 is a stronger base than PH3
The basicity of group 15 hydrides decreases as we move down the group. This is because the size of the central atom increases as we move down the group. As a result, the lone pair of electrons on central atom is not readily available for donation.
Nitrogen is the smallest member in group 15. Hence the lone pair of electrons on nitrogen can be easily donated. On the other hand, phosphorus is bigger in size than nitrogen and therefore the, lone pair of electrons is not readily available for donation. More readily a species donates an electron pair, more basic it is. Thus, NH3 is more basic than PH3.