Question

What is molecular geometry?

Answer 1

Molecular geometry is a way of describing the shapes of molecules. It applies a theory called VESPR for short.

VESPR stands for valence shell electron pair repulsion. This theory basically says that bonding and non-bonding electron pairs of the central atom in a molecule will repel (push away from) each other in three dimensional space and this gives the molecules their shape.

We can use the following notations when examining a Lewis structure of a molecule.
A = central atom
X = peripheral atoms
E = non-bonding electron pairs of the central atom

`AX_2` = linear molecule
`AX_3` = trigonal planar
`AX_3E` = trigonal pyrimidal
`AX_2E_2` = bent
`AX_4` = tetrahedral molecule

Here are some examples:

`H_2O` we need to consider the central atom of water which is oxygen. The oxygen has two bonding electron pairs (single bond to each H) and two non-bonding pairs giving water a `AX_2E_2` conformation and a bent shape.

`CO_2` is `AX_2` = linear molecule
`BH_3` is `AX_3` = trigonal planar
`NH_3` is `AX_3E` = trigonal pyrimidal
`CH_4` is `AX_4` = tetrahedral molecule