The VSEPR theory or model

The Valence Shell Electron Pair Repulsion (VSEPR) model:

•         is based on the number of regions of high electron density around a central atom (effective pairs).

•         can be used to predict structures of molecules or ions that contain only non-metals by minimizing the electrostatic repulsion between the regions of high electron density. Effective pairs must be as far as possible from each other.

•         can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.

•         does fail in some cases, that is there are exceptions

http://www.chem.purdue.edu/gchelp/vsepr/

The VSEPR rules

1. Draw the Lewis structure for the molecule or ion.

2. Count the effective pairs or total number of regions of high electron density (bonding and unshared electron pairs) around the central atom.

•         Double and triple bonds count as one effective pair (ONE REGION OF HIGH ELECTRON DENSITY).

•         An unpaired electron counts as one effective pair (ONE REGION OF HIGH ELECTRON DENSITY).

•         For molecules or ions that have resonance structures, you may use any one of the resonance structures.

                             http://www.chem.purdue.edu/gchelp/vsepr

Polar and non polar molecules

A molecule like HF is said to be dipolar or to have a dipole moment

Any diatomic molecule that has a polar bond will show a dipole moment

Polyatomic molecules with polar bonds

Examples of polar polyatomic molecules

Water                                                                   Ammonia

The dipoles of the bonds don’t cancel out so the molecule has a dipole moment.  These molecules are dipoles.

 Examples of Non-Polar Polyatomic molecules

Carbon Dioxide                                          Boron Trifluoride

                               Carbon Tetrachloride

The dipoles of the bonds cancel out because of the symmetric shape.

      These molecules are non-polar molecules