Both `overset(* *)NH_(3) " and " overset(* *)PH_(3)` are Lewis bases due to the presence of lone electorn pair on the central atom. However, `NH_(3)` is more basic than `PH_(3)`. The atomic size of nitrogen (Atomic radius = 70 pm) is less than that of phosphorus (Atomic radius=110 pm) As a result, electron density on the nitrogen atom is more than on phosphorus. This means that electron releasing tendency of ammonia is also more and is therefore, a stronger base than phosphine.
