Ans. In NH3 molecule, the central atom of nitrogen has three unpaired electrons in the ground state. The nitrogen atom then undergoes sp3 hybridization and forms four sp3 hybrid orbitals. The three sp3 hybrid orbitals overlap with three s orbitals of hydrogen atom while the fourth hybrid orbital contains lone pair of electrons and does not overlap. Thus the force of repulsion between lone pair and bond pair becomes greater than the force of two bond pairs of electrons. As a result, the bond between H-N-H decreases from 109.5o to 107.5o. hence, the geometry of the NH3 molecule becomes a trigonal pyramid.
