Question

What the shape of an ammonia molecule?

Answer 1

The ammonia molecule is a trigonal pyramid to a first approx., with `/_H-N-H` bonds angles `=` `104-5^@`.

Explanation:

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 `3xxN-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^@` angles between the electron pairs, the molecule is properly described as trigonal pyramidal. The `/_H-N-H` bonds angles are compressed from `109.5^@` bonds angles to approx. `104-5^@` because of the stereochemically active lone pair.

When ammonia reacts as a base upon reaction with water to give ammonium ion, `NH_4^+`, the `/_H-N-H` bonds angles become equal to `109.5^@`.