Ammonia (NH₃) has a trigonal pyramidal molecular geometry, with bond angles of about 107 degrees. Picture a three-sided pyramid: nitrogen sits at the top, and three hydrogen atoms form the triangular base below it. This shape results from four groups of electrons around the nitrogen atom, one of which is a lone pair that pushes the three hydrogen atoms downward and closer together.
Electron Geometry vs. Molecular Geometry
These two terms describe different things, and the distinction matters for understanding ammonia. Electron geometry accounts for all electron groups around the central atom, including lone pairs. Molecular geometry only considers where the actual atoms sit. In NH₃, nitrogen has four electron groups: three bonds to hydrogen and one lone pair. Four electron groups arrange themselves in a tetrahedron, so the electron geometry is tetrahedral.
But because one of those four positions is occupied by a lone pair rather than an atom, the shape you’d actually see (the molecular geometry) is a tetrahedron with one corner missing. That’s a trigonal pyramid. The nitrogen atom sits at the apex, and the three hydrogen atoms spread out below it in a triangular arrangement.
Why the Bond Angle Isn’t a Perfect 109.5°
In a perfect tetrahedron, like methane (CH₄), all bond angles are 109.5°. Ammonia’s H-N-H angle is smaller, around 107°. The reason is that lone pair electrons take up more space than bonding electrons. A bonding pair is shared between two atoms and gets pulled in both directions, keeping it relatively compact. A lone pair belongs entirely to the nitrogen atom and spreads out more broadly, crowding the three bonding pairs and squeezing them closer together.
This pattern holds across similar molecules. Methane has no lone pairs and bond angles of 109.5°. Ammonia has one lone pair and compresses to about 107°. Water has two lone pairs and compresses further to about 104.5°. Each additional lone pair pushes the bonding pairs a little closer, shrinking the angle by a few degrees.
Hybridization of the Nitrogen Atom
Nitrogen in ammonia is sp³ hybridized. This means one s orbital and three p orbitals on nitrogen blend together to form four equivalent hybrid orbitals, each pointing toward a corner of a tetrahedron. Three of these orbitals overlap with the hydrogen atoms to form bonds. The fourth holds the lone pair. This sp³ hybridization is what creates the underlying tetrahedral electron arrangement that gives ammonia its pyramidal shape.
Bond Length and Physical Dimensions
Each nitrogen-hydrogen bond in ammonia is exactly the same length: 1.012 angstroms, or about 101 picometers. That uniformity makes sense because all three bonds are identical, each connecting nitrogen to a single hydrogen atom with the same type of orbital overlap. The molecule is compact. If you placed it on the page, the nitrogen would sit slightly above the plane formed by the three hydrogens, creating a shallow pyramid rather than a tall, pointy one.
Why the Shape Makes Ammonia Polar
Geometry directly determines whether a molecule is polar, and ammonia’s pyramidal shape makes it distinctly so. Each N-H bond is polar on its own because nitrogen pulls electrons more strongly than hydrogen. In a perfectly symmetrical molecule, those individual polarities could cancel out. In ammonia, they don’t. The three bond dipoles all point upward toward nitrogen, and because the molecule is pyramidal rather than flat, they add together instead of canceling. The result is a net dipole moment of about 1.47 Debye, directed from the hydrogen end toward the nitrogen end.
This polarity is why ammonia dissolves readily in water, acts as a base by donating its lone pair, and has a higher boiling point than you’d expect for such a small, lightweight molecule. The shape isn’t just a geometry fact for a textbook. It determines how ammonia behaves chemically.
Comparing NH₃ to Similar Molecules
Ammonia belongs to a family of molecules that all start with the same tetrahedral electron arrangement but look different because of lone pairs:
- Methane (CH₄): Four bonding pairs, zero lone pairs. Shape is tetrahedral with bond angles of 109.5°. Nonpolar because the symmetry cancels all dipoles.
- Ammonia (NH₃): Three bonding pairs, one lone pair. Shape is trigonal pyramidal with bond angles of about 107°. Polar.
- Water (H₂O): Two bonding pairs, two lone pairs. Shape is bent with bond angles of about 104.5°. Even more polar than ammonia.
All three molecules have four electron groups and tetrahedral electron geometry. The difference in their visible shapes comes entirely from how many of those groups are lone pairs versus bonds to atoms. More lone pairs means more compression of bond angles and a less symmetrical shape.