Why ammonia is a stronger base than phosphine?

Explanation

The basic strength decreases ifl the following order : `NH_3` > `PH_3` > `AsH_3` > `SbH_3` > `BiH_3` Lewis basic strength is the ability to release the electron pair. Nitrogen has small size and high electron density. So, it is highly basic.The size of the central atom increases down the group i.e. from nitrogen to bismuth. Hence, the electron density decreases in the same manner. Consequently, the basic strength decreases down the group. Therefore, `NH_3` is a stronger lewis base than `BiH_3`.