Final answer:
In trigonal pyramidal molecules, the bond angles are slightly less than 109.5 degrees due to lone pair-bond pair repulsions, such as in ammonia with a bond angle of about 107 degrees.
Step-by-step explanation:
The bond angles present in a molecule with trigonal pyramidal geometry are slightly less than 109.5 degrees. This geometry can be thought of as a tetrahedron with one vertex missing, which would be the location of a lone pair of electrons. The presence of this lone pair causes lone pair-bond pair repulsions (LP-BP repulsions), which push the bonded atoms slightly closer together, reducing the bond angles from the idealized tetrahedral angle of 109.5 degrees. For example, in the case of ammonia (NH3), the actual bond angle is about 107 degrees due to these repulsions.