ROCO Chapter 4: Overview

The previous chapter defined electron density and illustrated several of its uses. This chapter concerns just one aspect of electron density: the calculation of partial atomic charges.

An atom’s charge is simply the sum of its component charges, the positive charge of the nucleus plus the negative charge of the atom’s electron density cloud. Unfortunately, the procedure for calculating the cloud’s charge is quite inconvenient, so atomic charge calculations are difficult (especially for large molecules).

This chapter describes a type of picture called an electrostatic potential map (or more commonly a potential map) that often provides information about atomic charges. Some uses of potential maps include:

Showing which parts of a molecular are polar (atoms carry significant partial charges) and nonpolar.
Comparing charge distributions in different molecules. These comparisons are useful for predicting the outcome of a chemical reaction.
Revealing irregularities in the shape of an atom’s electron cloud, showing regions where electron density is concentrated and regions where it is depleted.
Identifying charged atoms that attract (or repel) charged atoms in other molecules. Intermolecular interactions like these cause molecules to aggregate and form polymolecular complexes, liquids, and solids. These interactions also affect chemical reactivity.