Why do two compounds with the same generic formula NH3 and BH3 have different molecular geometries?

Answer 1

Boron is in group 13, nitrogen is in group 15.

In NH3 ammonia, the nitrogen atom achieves an octet , its four electron pairs are approximately tetrahedral

In the planar BH3 molecule there are only 6 electrons around the boron which is said to be electron deficient and the three covalent bonds are at 1200.

BH3 dimerises to form B2H6 which has bridging H atoms between the Boron atoms. This inceases the electron density on the B atoms but the octet is not achieved.

Answer 2

The difference in molecular geometries between NH3 (ammonia) and BH3 (borane) is due to the different hybridization states of the central atom. Nitrogen in NH3 forms four electron pairs using sp3 hybridization, resulting in a trigonal pyramidal shape. Boron in BH3 only forms three electron pairs using sp2 hybridization, leading to a planar shape.