Question

What is the shape of a molecule that has three single covalent bonds and one lone pair on the central atom?

Answer 1

The geometry of the electron pairs is tetrahedral to a first approximation..........

Explanation:

But the geometry of the molecule is trigonal pyramidal, inasmuch as we describe molecular geometry on the basis of atoms AND NOT electronic geometry.

So, we can take the ammonia molecule, `NH_3`, as an exemplar. There are `5+3=8" electrons"`, i.e. 4 electron pairs to distribute around the central nitrogen. There are `3xxN-H` bonds (that is 3 electron pairs), and one nitrogen-centred lone pair. The 3 bonding pairs, and the lone pair have tetrahedral geometry to a first approx. However, the lone pair tends to compress the `/_H-N-H` bonds from the ideal tetrahedral angle of `109.5` `""^@` to `104.5` `""^@`.

The lone pair on nitrogen is said to be `"stereochemically active"`, because it influences molecular geometry. And in fact, the lone pair is basic:

`H_3N: + H_2O rightleftharpoons H_4N^+ +""^(-)OH`,

And with the ammonium ion, the nitrogen centre is quaternized; it has donated its lone pair to form a fourth `N-H` bond. And thus the geometry of ammonium is tetrahedral with each `/_H-N-H=109.5^@`.