Why are the bond angles in ammonia not #109.5^@#?
Because nitrogen has a stereochemically active lone pair of electrons..........
Boron is a Group 3 atom, and has the 3 valence electrons. Simple
On the other hand, there are 4 electron pairs around the central nitrogen atom in ammonia:
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?