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