An example of trigonal pyramid molecular geometry that results from tetrahedral electron pair geometry is NH3. The nitrogen has 5 valence electrons and thus needs 3 more electrons from 3 hydrogen atoms to complete its octet. This then leaves a lone electron pair that is not bonded to any other atom. The three hydrogen atoms and the lone electron pair are as far apart as possible at nearly 109o bond angle. This is tetrahedral electron pair geometry.The lone electron pairs exerts a little extra repulsion on the three bonding hydrogen atoms to create a slight compression to a 107o bond angle.The molecule is trigonal pyramid molecular geometry because the lone electron pair, although still exerting its influence, is invisible when looking at molecular geometry. The molecule is three dimensional as opposed to the boron hydride case which was a flat trigonal planar molecular geometry because it did not have a lone electron pair.