Final answer:
Ammonia (NH3) has a trigonal pyramidal molecular geometry, with a tetrahedral electron-pair geometry that includes one lone pair and three hydrogen atoms bonded to nitrogen.
Step-by-step explanation:
The molecular geometry of NH3 (ammonia) is trigonal pyramidal. This structure occurs because the molecule includes four electron pairs around the central nitrogen atom, one of which is a lone pair, and three that form single bonds with hydrogen atoms. While the electron-pair geometry for ammonia is tetrahedral, the actual molecular geometry is defined by the positions of the atoms themselves, which forms a trigonal pyramid. The nitrogen atom is at the apex of this pyramid, and the hydrogen atoms form the base. The presence of the lone pair causes the H-N-H bond angles to be slightly less than the ideal 109.5° found in a perfect tetrahedron due to the lone pair occupying more space and causing greater repulsion.