What the shape of an ammonia molecule?

1 Answer
Sep 25, 2016

Answer:

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^@#.