Our primary goal for today is to be able to draw diagrams of molecules, such as we see in figure 1 .
Figure 1. A Few Small Molecules
These diagrams tell us that the F2 molecule has a single bond, the CO2 molecule has two double bonds, and the HCN molecule has one single bond plus one triple bond.
There are two main methods for constructing such diagrams.
- Part I of this document describes the smart way to do it. This involves counting the bonding and antibonding electrons, so we call it the BABE method. As a corollary of the BABE method, we have the hole counting method, which is particularly easy to use, and applies to a wide range of electron-rich molecules, including many organic molecules. We shall see that BABE is nicely consistent with theory, including Molecular Orbital theory.
Part I is meant to give a “clean” and self-contained explanation of BABE and of hole-counting. That means it is suitable for readers who have never heard of Lewis dot structures.
Part II provides background and context. In particular it contrasts the BABE
method with an older, widely known method, namely the “Lewis dot” method. It involves counting bonding and non-bonding electrons, so it could be characterized as BNBE. We shall see that the usual rationale for the Lewis method is incompatible with the available data, and incompatible with modern theory.
* PART I
2 How to Draw Molecules
In this section, we discuss a way of diagramming molecules that has many advantages. It is in all ways preferable to Lewis dot diagrams.
The diagrams provide predictions about the number of bonds. This is important, because it implies, among other things, predictions about the existence, non-existence, and reactivity of molecules. That is, when we learn there are four single bonds in CH4 and two double bonds in CO2. we also learn that reactions that produce molecules of CH4 are vastly more plausible than reactions that produce molecules of CH2. and similarly molecules of CO2 are more plausible than molecules of CO4 .
2.1 Electron-Rich Molecules
Our first goal is to draw (and understand!) some simple molecules. Before we start talking about rules, let alone theories, let’s look at some data.