One purpose of this guide is to show you how to describe a chemical reaction in terms of quantum mechanical wavefunctions. Reactions require the breaking and making of chemical bonds, so you shall first learn how to describe the wavefunction of a bond.
The first step is to define what we mean by "chemical bond". The best definition is an "operational" one:
A chemical "bond" is anything that holds two or more atoms together, usually within 1-3 Å of each other.
This definition says what a bond does, but it does not say how a bond comes about. A bond might be caused by any number of things ("electron sharing", "charge attraction", "instantaneous correlation"). The important thing is that all of these causes have the same effect, bond formation, and they can all be explained with the same theory, quantum mechanics.
Given what bonds do, it might seem that exotic conditions are required for bonding to occur, but this is not at all the case. Atoms are gregarious. They bond together in many different ways and in almost all situations (only inert gas atoms resist bonding). This helps explain why so many different types of bonding mechanisms exist.
"Gregariousness" does not imply that "anything goes". Many atoms seem to form bonds according to well-defined "rules". Carbon atoms, for example, usually make four bonds, while hydrogen atoms usually make only one bond. Recognition of these "rules" is useful because they make reliable predictions practical (without resorting to complex theories), and because they provide a large experimental body of evidence against which bonding theories can be tested.
Finally, it should be noted that the outcome of bonding is not always the same. Different bonds may have vastly different bond strengths, even when some of the same atoms are involved. For example, it takes 3.6 times as much energy to break a mole of HF molecules into H and F atoms (bond strength = 135 kcal/mol) as it does to break a mole of F2 molecules into F atoms (bond strength = 37 kcal/mol). Bond lengths can vary as well. Experimental measurements show that the longest CC bonds (over 1.6 Å) are 33% longer than the shortest CC bonds (~1.2 Å). A good bonding theory should be able to account for all of these observations.
(last updated 6/7/97)