i. In the BF3 molecule,
The central boron atom undergoes sp2 hybridization giving rise to three sp2 hybridized orbitals directed towards three comers of an equilateral triangle.
Thus,
The geometry is trigonal planar.
ii. In the NH3 molecule,
The central nitrogen atom undergoes sp3 hybridization giving rise to four sp3 hybridized orbitals directed towards four comers of a tetrahedron.
The expected geometry of NH3 molecule is regular tetrahedral with bond angle 109°28′.
There is one lone pair of electrons in one of the sp3 hybrid orbitals of nitrogen.
The lone pair-bond pair repulsion distorts the bond angle.
Hence,
The structure of NH3 is distorted and it has pyramidal geometry.
