Ammonia
NH3Same four electron groups as methane, but one is a lone pair. Look edge-on to see the pyramid, and compare the angle with methane's.
- N–H 1.02 Å
- ∠HNH 105°
Ammonia’s nitrogen has five valence electrons. Three of them pair up with hydrogens to make three N–H bonds, and the other two stay on nitrogen as a lone pair. That’s four electron groups, the same count as methane’s carbon, so nitrogen is sp³ hybridized too: three sp³ orbitals hold the N–H bonds and the fourth holds the lone pair.
The four electron groups point roughly toward the corners of a tetrahedron, but only three of them have atoms at the end. So the shape you see is a trigonal pyramid: nitrogen at the top, three hydrogens forming the base. Look at the model edge-on and the pyramid is easy to spot.
A lone pair is held by only one nucleus, so it spreads out more than a bonding pair and pushes the N–H bonds a little closer together. That’s why the H–N–H angle is smaller than methane’s 109.5°: about 107° in experiments. Turn on Angles & lengths to compare with the computed model.
That lone pair is also what makes ammonia a base: it’s an unshared pair of electrons sitting on the outside of the molecule, ready to grab a proton.
Sources
- OpenStax Organic Chemistry, 1.10: Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur
- OpenStax Chemistry 2e, 7.6: Molecular Structure and Polarity
3D structure, measurements and electron clouds computed with PySCF (B3LYP/def2-SVP). Computed values can differ from measured ones by a degree or so, or a few hundredths of an ångström.