The electron geometries of water and ammonia are both based on a tetrahedron. The central atom in ammonia is nitrogen which is surrounded by four electron pairs; three bond pairs and one lone pair. In water, the central atom is oxygen surrounded by four electron pairs also; two bond pairs and two lone pairs. The presence of four electron pairs on the central atom of each molecule means that their electron geometries are both tetrahedral according to Valence Shell Electron Pair Repulsion Theory.
However, due to the presence of lone pairs, the molecular geometry of water is bent(angular) while the molecular geometry of ammonia is trigonal pyramidal.
