What is molecular geometry?

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