Why do we have to memorize Lewis structures of ....

[nineteen]

Why do we have to memorize the structures of all the nitrogen oxides? Isn't there any way to understand about the Lewis structures of all the nitrogen oxides and their weird ways of making bonds?

 

[Christopher Grayce]

Goodness, you should certainly not be *memorizing* the Lewis structures of the oxides of nitrogen. The entire point of studying this system is that it is a compact illustration of all the big principles that go into writing Lewis structures, including a lot of the challenges to the whole idea.

It may be important to start off by emphasizing that Lewis structures are an *approximation* to what electrons actually do in molecules. Lewis structures are a reflection of G. N. Lewis's turn of the (20th) century idea, now called "valence bond theory," that explained chemical bonding as the result of atoms sharing pairs of electrons. Lewis had no idea *why* sharing pairs of electrons caused bonding, nor what was special about *pairs* of electrons, nor why this would explain so much about chemical valence (the ratios in which elements combine, e.g. C and O combine as CO and CO_2, and sometimes CO_3). He didn't have the modern quantum mechanical understanding of electrons in atoms and molecules to help -- those ideas were developed in the 1920s and 1930s, a decade or two after Lewis did his work.

So, not surprisingly, when quantum mechanics *did* come along, and we had a complete first-principles understanding of chemical bonding, Lewis's picture turned out to be not 100% complete or accurate. For one thing, electrons aren't shared by just two atoms in a molecule, they are always shared by *all* the atoms in the molecule. There's no way you can point to an electron and say "this electron belongs to this oxygen, and this other electron to this carbon." (If you've studied it, the Heisenberg uncertainty principle tells you why.)

Nevertheless, Lewis's ideas are *so* generally useful that chemists have been very reluctant to abandon them. Instead, what we have done is follow the lead of Lewis himself and use certain extensions to describe tricky systems using Lewis's language (and that of VB theory in general).

For example, "resonance." We discover molecules that can't be described accurately by one Lewis structure, but we invent the idea of a hybrid of several Lewis structures, and find the system *can* be described by these. We find that the octet rule all by itself isn't always enough to pick out the best Lewis structure, and so we invent the idea of "formal charge" and find that this allows us to mostly reliably identify the best Lewis structures, or identify the contributions of various resonance structures to the hybrid.

By using tricks like these, we are able to keep the basics of Lewis's simplification of the real underlying physics of chemical bonding alive, and that gives us a powerfully compelling visual language for describing bonding in molecules that does *not* require us to plunge into the complicated math of quantum mechanics every time we want to answer a simple question like "why is the hydrogen attached to THIS oxygen much more acidic than the hydrogen attached to this other?"

And the reason you are studying the oxides of nitrogen system is that most of the tricks we need to use to extend Lewis's ideas to most molecules are on display here. If you master this system, you'll have mastered the tricks.