Shapes of Molecules and VSEPR Theory

Dr. Walt Volland revised April 2004

After you have studied this lesson you should be able to:

  • give a definition and description of a ball and stick model for molecules
  • describe the valence shell electron-pair repulsion VSEPR model
  • describe and show what is meant by bond angles
  • sketch examples of linear, planar triangular, bent, tetrahedral and pyramidal structures
  • tell what bond angels are for molecules based on VSEPR models when given formulas

 

 

The valence shell electron-pair repulsion model, VSEPR, model attempts to predict molecular shapes. The model assumes that electron pairs around a central atom repel one another. Repulsions between the electron pairs force definite positions based on the number of "groups" or electron clouds.

The model deals with counting groups of electrons radiating out from a "central atom" . The model is can make accurate predictions. We will deal only with central atoms that have from two, three or four groups of electrons radiating from the central atom.

The process of determining the structure uses the following steps.

The prediction's first step is to draw the Lewis dot structure.

The second step is to count the number of electron clouds radiating out from the central atom.

The three types of basic structures are shown here.NOTE the electron pairs are also equivalent to "groups" of electrons such as lone pairs, double bonds and triple bonds. The central atoms shown here are the building blocks for huge molecules. The angles and shapes in DNA can be predicted using the same VSEPR style model.

The simplest structure has two groups around the central atom. Carbon monoxide, CO2, is an example of this kind of shape. The two groups of electrons in the two double bonds repel each other and assume a shape with an angle of 180 degrees.

The shape for BeCl2 is the same because there are two groups of electrons around the Be atom.

 

 

Examples of molecules and shapes

bent the lone pair is not shown

angles are 120 degrees between bonds

trigonal planar

angles are 120 degrees between bonds

trigonal pyramid

angles are roughly 109.5 degrees between bonds