Predicting Shapes of Molecules using VSEPR Theory

Methane and sample Exercise

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 counts groups of electrons radiating out from a "central atom" . You can use the model to make reasonably accurate predictions. We will deal only with central atoms that have either two, three or four groups of electrons radiating from the central atom.

MAJOR IDEA:
Combinations of atoms with the same number of atoms and electrons will have the same type of Lewis structures. Carbon monoxide, CO, and nitrogen. N2,both have ten velence electrons and two atoms. They both have a triple bond. We expect CN1- cyanide ion with 10 valence electrons to also have a triple bond.

The three step process of determining the shape of a molecule is illustrated below.

1. First draw the Lewis dot structure.

CH4 has the Lewis structure

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

3. The third step is to select the shape that matches the count for electron cloud groups. The methane molecule is tetrahedral with four groups all single bonds.

 

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 basic shapes for the electron clouds are linear, triangular planar, and tetrahedral. Note the bond angles, actual angles will be slightly diferent because of properties of the atoms in the molecule. The actual angles will be reasonably close. This is an only an approximate model.

NOTE electron pairs and cloud groups are used to mean the same thing.

Exercise: What is the VSEPR category for BeH2 and HCN? Draw the Lewis structure and predict shape and the bond angles.

Click for answer: