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Okay, thank you. I'm very glad to be here. I should say by way of introduction that this lecture will last two hours,

but the good news is there will be a break after one hour for various human needs and for people to leave as they have to leave.

And I will talk fast. I start out slow, but I will in the end wind up talking very fast.

So, let's talk about bonding. And here are two quotations to begin with.

From two great theoreticians, Charles Coulson and Robert Mullican.

These quotations are 60 years old, and you can read at the top.

This is Coulson captured in an after-dinner talk that he gave about a very wise comment about the chemical bond.

The point is here that it is in some ways a theoretical concept.

The second statement by Mullican is very typical of Mullican,

who always said things in a way that were full of qualifications and saw things as complicated, and yet introduced some simple ideas.

The idea is old, or the concern about bonding is old. So here is a small history.

And for a more careful reading, you should look at, you cannot see with this pointer, but it doesn't matter,

the books that are at the bottom by Nye and especially Alan Rock's masterful history of the middle of the 19th century in chemistry.

It begins with the idea of an element with Dalton's atom. Notice Lavoisier does not figure here.

Though at the beginning of modern chemistry, in the idea of a chemical bond, he does not enter so much.

Then Auguste Laurent, the idea of a molecule, and then an explosion in the middle of the 19th century.

With first time chemical structures being drawn in the way that we expect them to see, with Kauper and Crumb Brown,

Butlorov's chemical structures, and of course the seminal work of Kekulé.

It is in about ten years, those years that we begin to see structures that look like structures we recognize.

Then we have the movement into three dimensions with van't Hoff and Lebel.

In the time of the first war, the independently developed theories of Lewis and Koestel on bonds as shared electron pairs.

And around the same time, during the first world war, our first idea of the metrics of distances between atoms.

Then quantum mechanics comes in, the Heitler and London calculation, and then Pauling and Mulliken.

Let me show you some structures. I see that I have to be nearer to this for this to work.

So here is a collage of structures from the 19th century.

So what you see here is, some things were already like ours.

From about 1830, the symbols for the elements were unified through the great authority of Berzelius.

And so we see carbon and C and H as standing for carbon and hydrogen.

What was not yet unified, as you see in these formulas over here at left, is whether you should put the subscript that we now see as a subscript.

Some people put it as a superscript. It was OK. No one had agreed on how to put them.

What people were desperately trying to do, and you can see it in these, is to somehow communicate to people, this is hanging together with something else.

So that's what all those parentheses of various kinds are.

And there were models at lower left, the models by August Hoffman.

And they are perfectly good models, these date to 1865, that we recognize today.

The only thing about them, because this is 1865 and before van't Hoff and Lebel, is that the carbons are absolutely square planar.

Which is just fine, because that's not what he was trying to communicate with geometry.

He was just trying to communicate what's connected to something else.

And that is communicated perfectly well.

At the top, in the middle, you see Kekulé's worst formal, which are these structures for, so there is a structure for methyl chloride.

So what are these? There is this little bar, and there's an actual model down below, which comes from Kekulé's laboratory.

What he's trying to communicate is not atomic mass, which was known around that time fairly well.

He's trying to communicate by that bar for the sea with four bulges.

He is trying to communicate that carbon has four valences.

And as you see in the CH, the hydrogens and the chlorine below, they also, and they only have one bulge.

So they are only one valence, and it's connected to the carbon up above.

Here is, in this model, how Kekulé, within 10 years, and this is traced in Rock's book, had about four models for benzene.

Only the last one of which is the one that we know.

Here is Kekulé, one way, with these worst formal that Kekulé communicated the structure of benzene.

So here you see, if you kill the lights a little bit, you could see it better, but you see these things with four balls in a row.