Explain why NH₃ is pyramidal but BF₃ is trigonal planar in structure ???

Dear Student,

We can determine shape of a molecule by knowing hybridization of the central atom in it.

Hybridization of N in in NH₃ is sp³. So its shape should be tetrahedral, but its actual shape is pyramidal.

The four pairs of electrons (three bonding pairs and one non-bonding lone pair) repel each other, giving the molecule its pyramidal shape. The H–N–H bond angle decreases from 109.5 degrees to 107 degrees to minimize the lone pair-bond pair repulsion.

But in BF₃, hybridization of B is sp². As we know that, for sp² hybridization, shape of the molecule is trigonal planar. So BF₃ is trigonal planar in structure.