According to Lewis Theory of Acids and Bases, an electron pair donor is termed a base and an electron pair acceptor is termed an acid. Higher the electronegativity, lower will be the tendency for an atom to donate its un-shared pair of electrons. It is observed that phosphine (PH3) is less basic than ammonia (NH3) in spite of the fact that N is more electronegative than P. Why?
Answer:
The bond angle is PH3 is close to 90o which indicates that three of the hybrid orbital forming bonds with hydrogen atom have become almost identical to pure p-orbital and the lone pair has almost become spherical and now resembles an s-orbital. As the electron density in the s-orbital is close to the nucleus, it is difficult to distort or withdraw electron density from an s-orbital. However in NH3, the lone pair still is a sp3 orbital and electron density in it can easily be distorted or withdrawn compared to an s-orbital. So, PH3 is less basic than NH3.
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