NH3 electron geometry
The electron geometry of \( \mathrm{NH_3} \), ammonia, is tetrahedral. Nitrogen is the central Group 5A atom, and the Lewis structure contains four regions of electron density around nitrogen: three \( \mathrm{N-H} \) bonding pairs and one lone pair. VSEPR theory counts both bonding pairs and lone pairs when assigning electron geometry.
Ammonia has tetrahedral electron geometry but trigonal pyramidal molecular geometry. Electron geometry describes all electron domains around nitrogen, while molecular geometry describes the positions of the atoms only.
Valence electrons in ammonia
Nitrogen contributes five valence electrons because it is in Group 5A. Each hydrogen contributes one valence electron. The total number of valence electrons in \( \mathrm{NH_3} \) is:
\[ 5 + 3 \cdot 1 = 8 \]These eight valence electrons form three single \( \mathrm{N-H} \) bonds and one lone pair on nitrogen. Each single bond contains two electrons, so the three bonds account for six electrons:
\[ 3 \cdot 2 = 6\ \text{bonding electrons} \]The remaining two electrons form the lone pair on the nitrogen atom:
\[ 8 - 6 = 2\ \text{nonbonding electrons} \]Lewis structure and electron-domain count
In the Lewis structure of ammonia, nitrogen is placed at the center because hydrogen can form only one bond. Nitrogen forms three single covalent bonds with hydrogen atoms and retains one lone pair. Around the nitrogen atom, VSEPR theory counts each bond as one electron domain and each lone pair as one electron domain.
| Feature around nitrogen | Number in \( \mathrm{NH_3} \) | Electron-domain contribution |
|---|---|---|
| \( \mathrm{N-H} \) bonding pairs | 3 | Three bonding domains |
| Lone pairs on nitrogen | 1 | One nonbonding domain |
| Total electron domains | 4 | Tetrahedral electron-domain arrangement |
VSEPR classification
VSEPR notation for ammonia is \( \mathrm{AX_3E} \). The symbol \( \mathrm{A} \) represents the central atom nitrogen, \( \mathrm{X_3} \) represents three bonded hydrogen atoms, and \( \mathrm{E} \) represents one lone pair on nitrogen.
| VSEPR feature | Value for \( \mathrm{NH_3} \) | Geometric meaning |
|---|---|---|
| Central atom | \( \mathrm{N} \) | Nitrogen is the atom surrounded by the electron domains. |
| Bonded atoms | 3 hydrogen atoms | Three \( \mathrm{N-H} \) bonds occupy three positions around nitrogen. |
| Lone pairs on central atom | 1 lone pair | The lone pair occupies space and affects bond angles. |
| Electron-domain geometry | Tetrahedral | Four total electron domains arrange themselves as far apart as possible. |
| Molecular geometry | Trigonal pyramidal | The visible atom positions form a pyramid with nitrogen at the apex. |
Bond angle in ammonia
A perfect tetrahedral arrangement has bond angles of approximately \( 109.5^\circ \). In ammonia, the lone pair on nitrogen repels bonding pairs more strongly than bonding pairs repel one another. This stronger lone-pair repulsion compresses the \( \mathrm{H-N-H} \) bond angle.
\[ \text{ideal tetrahedral angle} \approx 109.5^\circ \] \[ \text{actual } \mathrm{H-N-H} \text{ angle in } \mathrm{NH_3} \approx 107^\circ \]The angle remains close to tetrahedral because the electron-domain geometry is still based on four regions of electron density. The difference arises because one region is a lone pair rather than a bonded atom.
Electron geometry compared with molecular geometry
Electron geometry and molecular geometry are closely related but not identical for molecules with lone pairs. In \( \mathrm{NH_3} \), the electron geometry includes the lone pair, so the arrangement is tetrahedral. The molecular geometry ignores the lone pair as a visible atom position, so the shape is trigonal pyramidal.
| Geometry type | What is counted | Result for \( \mathrm{NH_3} \) |
|---|---|---|
| Electron geometry | All bonding pairs and lone pairs around nitrogen | Tetrahedral |
| Molecular geometry | Only atom positions around nitrogen | Trigonal pyramidal |
Polarity and structure
Ammonia is polar. The three \( \mathrm{N-H} \) bonds are polar because nitrogen is more electronegative than hydrogen, and the trigonal pyramidal shape prevents the bond dipoles from canceling. The lone pair also contributes to the asymmetric electron distribution around nitrogen.
Common pitfalls
A common error is assigning trigonal pyramidal as the electron geometry of \( \mathrm{NH_3} \). Trigonal pyramidal is the molecular geometry. The electron geometry remains tetrahedral because VSEPR counts the lone pair as one of the four electron domains around nitrogen.