Although successive replacement of bonded atoms X by nonbonding electron pairs does not change the arrangement of electron pair domains about atom
A, the shape descriptor does change, as seen below for the AX
3 and AX
4 cases where X is an atom and E is a nonbonding electron pair.
| AX3 | AX2E | AX4 |
 |  |  |
| trigonal planar | bent | tetrahedral |
| AX3E | AX2E2 |
 |  |
| trigonal pyramid | bent |
According to VSEPR theory, the shape of a molecule
AXn depends on the total number of bonding and nonbonding electron pairs at
A.
The shape can be deduced from the Lewis structure for the compound. Thus the first step is to use the formula to draw the Lewis structure according to the guidelines given in the Lewis structure module.