What the shape of an ammonia molecule?

1 Answer
anor277
Sep 25, 2016

The ammonia molecule is a trigonal pyramid to a first approx., with /_H-N-H∠H−N−H bonds angles == 104-5^@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-H3×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 NN, with 109.5^@109.5∘ angles between the electron pairs, the molecule is properly described as trigonal pyramidal. The /_H-N-H∠H−N−H bonds angles are compressed from 109.5^@109.5∘ bonds angles to approx. 104-5^@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^+NH+4, the /_H-N-H∠H−N−H bonds angles become equal to 109.5^@109.5∘.

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