Dr. Gutow's Molecular Shapes Web Site

Introduction

The Model (VSEPR)

Examples

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.

Below is a Summary Table of the categories and names for the various sub-shapes.

Number of Groups	Basic Shape (click links below to see examples)	Sub-Shapes A = central atom X = atom attached to central atom E = nonbonding electron group on central atom
Diatomic (any number)	Linear Diatomic	Generic Formula AX	Shape Name (this refers to the atom arrangement, ignoring the lone pairs) Linear
2	Linear Triatomic	AX2	Linear
3	Trigonal Planar	AX3 AX2E	Trigonal Planar Bent
4	Tetrahedral	AX4 AX3E AX2E2	Tetrahedral Trigonal Pyramidal Bent
5	Trigonal Bipyramidal	AX5 AX4E AX3E2 AX2E3	Trigonal Bipyramidal Seesaw T-shaped Linear
6 Last Table Update: November 18, 2016 Expires: --- J. Gutow	Octahedral	AX6 AX5E AX4E2	Octahedral Square Pyramidal Square Planar