NH₃ Lewis Structure (Ammonia)

On the central nitrogen, count electron domains the VSEPR way: each bonded neighbor is one domain (a double or triple bond still counts as one), and each lone pair is one more. NH₃ has 3 bonding domains and 1 lone pair (AX₃E), for 4 electron domains in total.

Those domains arrange to minimize repulsion around the nitrogen, so the molecule is trigonal pyramidal with a bond angle of ≈107°. Lone pairs on nitrogen take space but do not count in the shape name, so the molecular geometry (trigonal pyramidal) differs from the electron geometry (tetrahedral).

NH₃ is VSEPR AX₃E: nitrogen is the central atom (A), the three N–H bonds are the bonding domains (X₃), and nitrogen keeps one lone pair (E). Four domains need about 109.5° of space, so the electron geometry is tetrahedral. Only the three hydrogens count in the shape, so the molecular geometry is trigonal pyramidal, with an H–N–H angle of about 107°. The same four-domain count appears next to methane and water: CH₄ is AX₄ with no lone pairs (tetrahedral molecule), NH₃ is AX₃E with one lone pair (trigonal pyramidal), and H₂O is AX₂E₂ with two lone pairs (bent). Lone pairs change the shape name; they do not change the four-domain electron geometry.

Why Is NH₃ Trigonal pyramidal?

Why is the electron geometry tetrahedral while the molecular geometry is trigonal pyramidal? Four electron domains surround nitrogen: three N–H bonding pairs and one lone pair. Those domains arrange in a tetrahedral pattern. The lone pair occupies one corner of that tetrahedron, so only the three hydrogens remain as atoms in the shape — a pyramid with a triangular base. Electron geometry counts every domain around nitrogen; molecular geometry counts only the atom positions. That is why ammonia is called both tetrahedral (electron geometry) and trigonal pyramidal (molecular shape).

NH₃ Bond Angle

The H–N–H bond angle is about 107°. The ideal tetrahedral angle is 109.5°. In ammonia the lone pair on nitrogen repels the bonding pairs more strongly than bonding pairs repel each other, so the angle compresses from 109.5° toward 107°. Lone-pair–bonding-pair repulsion is stronger than bonding-pair–bonding-pair repulsion — that ordering is why NH₃ sits between CH₄ (109.5°, no lone pairs) and H₂O (104.5°, two lone pairs).

NH₃ is polar because its trigonal pyramidal shape prevents the N–H bond dipoles from canceling.

Each N–H bond is polar: nitrogen (electronegativity 3.04) pulls shared electrons harder than hydrogen (2.20), so every bond dipole points toward nitrogen (δ− on N, δ+ on each H). Polar bonds are not enough by themselves. If NH₃ were planar and perfectly symmetric with no lone pair, the three equal dipoles could cancel in the plane. Nitrogen keeps one lone pair (VSEPR AX₃E), so the shape is trigonal pyramidal instead. The three bond dipoles share a component along the lone-pair axis; those components add, and a net molecular dipole remains pointing toward the nitrogen / lone-pair side.

CH₄ has four identical C–H bonds in a tetrahedral AX₄ shape with no lone pair, so its bond dipoles cancel and methane is nonpolar. Both molecules have four electron domains around the central atom, but only NH₃ has a lone pair — and that is why ammonia is polar while methane is not.

In general chemistry, nitrogen in NH₃ is described as sp³-hybridized: three N–H bonds and one lone pair give four electron domains. One 2s and three 2p orbitals mix into four hybrids — three form the N–H σ bonds, and one holds the lone pair — which matches the tetrahedral electron geometry. The measured H–N–H angle (~107°) is slightly tighter than the ideal 109.5° because the lone pair compresses the bonding pairs.