What is the shape of a molecule that has three single covalent bonds and one lone pair on the central atom?
The geometry of the electron pairs is tetrahedral to a first approximation..........
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,
The lone pair on nitrogen is said to be
And with the ammonium ion, the nitrogen centre is quaternized; it has donated its lone pair to form a fourth