Why the shape matters
The nitrogen lone pair is chemically active.
It influences the H–N–H bond angles and provides electron density that can accept a proton or coordinate to a Lewis acid.
Molecular compound · nitrogen hydride
Ammonia
NH₃
Ammonia is a molecular compound of nitrogen and hydrogen. Its nitrogen atom carries one lone pair, so NH₃ is trigonal pyramidal rather than flat. The same lone pair makes ammonia a Brønsted base and a Lewis base.
Everyday distinction: household “ammonia” is usually an aqueous solution containing dissolved NH₃ and related acid–base species, not a bottle of pure liquid ammonia.
Essentials
NH₃ is a small three-dimensional molecule, not a flat triangle.
A simple 3D drawing shows the important fact first: three N–H bonds form a pyramid around nitrogen. The lone pair occupies the fourth electron-domain direction and changes both geometry and reactivity.
Detail
The gas-phase H–N–H angle is about 106.7°.
Experimental gas-phase geometry places the H–N–H angle close to 106.7°. That value is smaller than the ideal tetrahedral angle because one of the four electron domains around nitrogen is a lone pair rather than a bond.
Model versus measurement
Tetrahedral electron domains do not imply a tetrahedral molecular shape.
Four electron domains surround N, but only three are bonds to atoms. Molecular geometry names the positions of atoms, so the result is trigonal pyramidal.
Understand
In water, ammonia behaves as a weak base.
The nitrogen lone pair can accept a proton from water. The equilibrium produces ammonium and hydroxide, but only a fraction of dissolved ammonia is protonated at ordinary concentrations.
Two related species
NH₃ and NH₄⁺ are not interchangeable.
Ammonia is neutral and has a lone pair. Ammonium carries a +1 charge and has four N–H bonds; protonation changes geometry and chemical behaviour.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | NH₃ |
|---|---|
| CAS Registry Number | 7664-41-7 |
| PubChem CID | 222 |
| Standard InChI | InChI=1S/H3N/h1H3 |
Advanced
Push past the first model without losing the simple picture.
Can ammonia hydrogen-bond?
Yes. N–H bonds can donate hydrogen bonds and the nitrogen lone pair can accept them. The interactions are generally weaker than the strong water–water hydrogen-bond network.
Is aqueous ammonia literally NH₄OH molecules?
Writing NH₄OH as if it were a discrete molecular solute is misleading. Aqueous ammonia is better described by NH₃, H₂O, NH₄⁺ and OH⁻ connected by acid–base equilibrium.
Why is ammonia a Lewis base?
The nitrogen lone pair can be donated to an electron-pair acceptor to form a coordinate bond.
Constituent elements