# What the shape of an ammonia molecule?

The ammonia molecule is a trigonal pyramid to a first approx., with $\angle H - N - H$ bonds angles $=$ $104 - {5}^{\circ}$.
How did we predict this shape? We know the ammonia molecule has 8 electrons to distribute nitrogen: one pair of electrons comprise the nitrogen-centred lone pair, and the other 6 electrons constitute the $3 \times N - H$ bonds.
There are thus 4 electrons pair to distribute around nitrogen. And their arrangement in a tetrahedron is the first approximation of the molecular structure. But we describe molecular shape on the basis of atomic geometry, not electronic geometry. While the 4 electron pairs are distributed as a tetrahedron about $N$, with ${109.5}^{\circ}$ angles between the electron pairs, the molecule is properly described as trigonal pyramidal. The $\angle H - N - H$ bonds angles are compressed from ${109.5}^{\circ}$ bonds angles to approx. $104 - {5}^{\circ}$ because of the stereochemically active lone pair.
When ammonia reacts as a base upon reaction with water to give ammonium ion, $N {H}_{4}^{+}$, the $\angle H - N - H$ bonds angles become equal to ${109.5}^{\circ}$.