The electrons around the atoms in a molecule repel each other.  They move to be as far apart as possible while still maintaining the bonding within the molecule.  The procedure for using the model is as follows:

1. Determine the correct Lewis structure for the molecule.  If it is a diatomic (has only two atoms) it is linear and falls into the AX category.  If it has 3 or more atoms continue with step 2.
   
2. Count the number of electron groups around the central atom.  A group of electrons is a bond, a nonbonding electron pair, or occasionally an unpaired nonbonding electron.  Each triple or double bond counts as only one group for the purposes of this model.
3. Based on this number of groups around the central atom the molecule falls into one of six basic categories. Within each category there are a number of different names for the shapes depending upon the number of atoms and nonbonding groups around the central atom.
   

Number of Groups around central atom	Shape when all groups are bonded atoms Electron geometry and molecular geometry are the same (click links to see examples you can manipulate)	Sub-Shapes (molecular geometry) A = central atom, X = atom attached to central atom, E = nonbonding electron group on central atom (click links to see examples you can manipulate)
Diatomic (any number of groups)	Linear Diatomic	Linear (AX)
2	Linear Triatomic	Linear (AX2)
3	Trigonal Planar	Trigonal Planar (AX3) Bent (AX2E)
4	Tetrahedral	Tetrahedral (AX4) Trigonal Pyramidal (AX3E) Bent (AX2E2)
5	Trigonal Bipyramidal	Trigonal Bipyramidal (AX5) See-Saw (AX4E) T-Shaped (AX3E2) Linear (AX2E3)
6	Octahedral	Octahedral (AX6) Square Pyramidal (AX5E) Square Planar (AX4E2)