Upon addition of a strong Lewis base, NH3 to BBr3 the bond angles between the ligands are pushed towards 109 degrees. The bond angles between BF3 and NH3 in the same reaction remain close to 120 degrees. Explain these observations.
The bond angle in molecules has to do with the hybridization of the central atom. In NH3, the molecule is sp3 hybridized. 1 s and 3 p atomic orbitals combine to form 4 sp3 hybridized molecular orbitals. 3 of these are bonds to Hydrogen. The 4th is occupied by the lone pair of electrons on nitrogen. Normally, as in CH4, the bond angles would be 109.5 degrees, but due to the non-bonding lone pair on the nitrogen, which because it is non-bonding (to another atom), has a higher electron density and pushes the 3 bonding MO's (molecular orbitals) away from it, making the bond angles smaller (approx 107).