NH3 electron geometry is tetrahedral because nitrogen is surrounded by four electron domains: three N–H bonding pairs and one lone pair.
Lewis structure and electron domains
Valence-electron counting places nitrogen as the central atom. Nitrogen contributes 5 valence electrons and the three hydrogens contribute 3 more, for a total of \(5 + 3 = 8\) valence electrons (4 electron pairs).
Three N–H single bonds account for 6 electrons (3 bonding pairs). The remaining 2 electrons form one lone pair on nitrogen. Nitrogen reaches an octet (6 electrons in bonding + 2 in the lone pair), and each hydrogen attains a duet through its single bond.
Electron domains around N in NH3
Bonding domains: 3 (three \(\sigma\) bonds N–H)
Nonbonding domains: 1 (one lone pair on N)
Steric number: \(SN = 3 + 1 = 4\)
Electron geometry versus molecular geometry
Electron geometry describes the spatial arrangement of all electron domains (bonding pairs and lone pairs) around the central atom. With \(SN = 4\), the electron geometry is tetrahedral.
Molecular geometry describes the arrangement of atoms only (the bonded nuclei). In NH3, one of the tetrahedral directions is occupied by a lone pair, leaving three hydrogen positions. The molecular geometry is therefore trigonal pyramidal.
VSEPR notation expresses this as AX3E: one central atom (A), three bonded atoms (X3), and one lone pair (E). An ideal tetrahedral angle is \(109.5^\circ\), while the H–N–H angle in ammonia is smaller (commonly quoted near \(107^\circ\)) because lone pair–bond pair repulsion is stronger than bond pair–bond pair repulsion.
Hybridization language
The \(SN = 4\) description aligns with an sp3 hybridization picture for nitrogen: four hybrid orbitals oriented approximately tetrahedrally, three forming \(\sigma\) bonds to H and one holding the lone pair. The lone pair’s greater electron density tends to compress the H–N–H angles below \(109.5^\circ\).
Quick reference table
| Feature | NH3 value | Meaning |
|---|---|---|
| Electron domains on N | 4 | 3 bonding pairs + 1 lone pair |
| Electron geometry | Tetrahedral | Arrangement of all electron domains |
| Molecular geometry | Trigonal pyramidal | Arrangement of the three H atoms |
| VSEPR label | AX3E | One lone pair on the central atom |
| Bond angle | \(\approx 107^\circ\) | Smaller than \(109.5^\circ\) due to lone pair repulsion |
| Polarity | Polar | Trigonal pyramidal shape prevents dipole cancellation |
Spatial picture
Common pitfalls
- Electron geometry versus molecular geometry confusion: tetrahedral describes domains, trigonal pyramidal describes atom positions.
- Planar drawings on paper: a flat Lewis structure sketch does not imply trigonal planar geometry.
- Lone pair omission: removing the lone pair from the domain count changes the predicted geometry incorrectly.
- Bond-angle expectation of \(109.5^\circ\): lone pair–bond pair repulsion commonly shifts NH3 angles toward \(107^\circ\).