Ammonia shape [ENDORSED]

[Alicia Yu 1A]

I know we talked about this in lecture, but I am still slightly confused on why NH3 is trigonal pyramidal?

[Chem_Mod]

As I discussed in class we name the shape of molecules based on the position of the atoms in the molecule.

In ammonia the three H atoms form the base of a trigonal pyramid and the N atom is the top of the pyramid.
Hence the name trigonal pyramidal.

[304984981]

NH3 has 4 regions of electron density, 3 of them are bonding pairs and 1 of them are lone-pairs. AX3E1

Although it is arrangded tetrahedrally, but the way to minimize the repulsion betweena atoms rearrange the atoms to form a TRIGONAL PYRAMIDAL shape

Why we call it pyramidal? since it looks like a pyramid! With bond angles(107) actually smaller than the tetrahedral one(109.5). It's because the repulsion strength between lone pairs is strongly than that between bonding pairs. As a result, the bonds are squeezed to a smaller angle.

On the graph we can see, the 107 degree angle is originally 109.5 degree but it was squeezed by the long pairs on the top of the Nitrogen atom.