Ammonia (NH3) is more basic than phosphine (PH3)
According to Lewis concept we know that, the substance which donates electron & accepts proton is known as base. Following, ammonia & phosphine both are basic compounds, because; nitrogen (N) of ammonia molecule, Phosphorus (P) of PH3 molecule contains one lone pair electron, which they can donate.
N & P are the elements of same group but the size of N atom is smaller than that of P. For that reason, the electro negativity of nitrogen is more than that of P. So, N attracts the electron of N-H bond towards itself. As a result the density of electron in N atom is high. So it can donate electron easily. So, its basicity is high.
On the otherhand, in case of phosphine, the electro negativity difference between P and H is less. So, P cannot attract the P-H bond electron towards itself. So, the density of electron in P is less. For that reason, it cannot donate electron so much easily like N. That’s why; ammonia is more basic than phosphine.
More electron density at N atom