Question

Why are the bond angles in ammonia not `109.5^@`?

Answer 1

Because nitrogen has a stereochemically active lone pair of electrons..........

Explanation:

Boron is a Group 3 atom, and has the 3 valence electrons. Simple `"VSEPR"` predicts that the 3 bonds of `BBr_3`, composed of 6 electrons, i.e. 3 electron pairs, arrange themselves in a trigonal plane to MINIMIZE the interaction between the bonding pairs.

On the other hand, there are 4 electron pairs around the central nitrogen atom in ammonia: `3xxN-H` bonds, and one nitrogen-centred lone pair of electrons. The lone pair is stereochemically active, and the most stable electronic geometry around the nitrogen is tetrahedral.

But we describe molecular geometry on the basis of atoms, NOT on lone pairs. So while electronic geometry is tetrahedral to a 1st approx. bonding geometry is trigonal planar. Are we clear?

The `/_H-N-H` `-=` `104-105^@`, rather than `109.5^@` because the nitrogen-centred lone pair, (which is of course close to the nitrogen atom), compresses the `/_H-N-H` bond angle.