TEACHING THE VSEPR MODEL AND ELECTRON DENSITIES

Abstract

This paper gives a simple pictorial introduction to the VSEPR model of molecular geometry and its physical basis: the Pauli exclusion principle. The treatment of the VSEPR model range from a simple empirical set of rules to help freshmen students predicting the geometry of molecules to more advanced treatments based on a careful analysis of the electron density (for advanced undergraduate or graduate students). The density can be readily calculated using ab intio or density functional theory (DFT) methods and it can also be obtained experimentally by X-ray crystallography. Unlike an orbital model of a molecule, the electron density is a physical observable. There are therefore advantages in interpreting the electron density to obtain information about bonding, advantages which are not as widely appreciated as they deserve to be. We also give a simple introduction to the quantum theory of Atoms in Molecules (AIM) and of its analysis of the electron density. We show how it provides a clear, rigorous and unambiguous definition of an atom in molecule that can be used as the basis for calculating the charge of the atom and indeed any of its other properties. [Chem. Educ. Res. Pract. Eur.: 2001, 2, 73-90]