The bond angles are compressed because the lone pair occupies an electron domain and repels the three bonding pairs more strongly than bonding pairs repel one another. Although the four domains retain an approximately tetrahedral arrangement, this unequal repulsion pushes the bonded atoms closer together. In ammonia, the resulting angle is often described as about 107°.
The electron-domain arrangement includes both bonding pairs and the lone pair, so it is approximately tetrahedral. Molecular geometry, by contrast, describes the positions of the bonded atoms and does not treat the lone pair as an atom in the visible framework. This distinction allows a Lewis formula to predict the three-dimensional molecular shape rather than only the number of bonds.
Because the bonded atoms occupy a three-dimensional arrangement around the central atom while the lone pair remains part of the electron-domain pattern, the charge distribution is not represented as a flat, symmetric triangle. This spatial asymmetry helps explain why ammonia is molecularly polar, linking geometry to its intermolecular interactions.
Recognizing the lone pair is important because it changes the arrangement from a simple three-bond pattern and alters the bond angles. That three-dimensional structure supplies information used to interpret molecular behavior, including spectroscopy and chemical reactivity. Thus, geometry is not merely a drawing convention; it connects electron-domain arrangement with observable chemical properties.
Start with the Lewis formula and identify the central atom, the three bonded atoms, and the lone pair on the central atom. Count these as four electron domains, then use their approximately tetrahedral arrangement to infer the molecular shape. Finally, account for stronger lone-pair repulsion when estimating compressed bond angles.
When describing a molecule with this geometry, distinguish the electron-domain arrangement from the molecular geometry, note the presence of the central-atom lone pair, and indicate that bond angles are compressed relative to an ideal tetrahedral relationship. Including these features communicates both the structural model and the reason it can help explain polarity and other molecular behavior.