Linus Pauling: In this lecture, the second of our series on valence and the electronic structure
of atoms, molecules and crystals, I shall talk about the covalent bond and the shared
electron pair bond – they are the same thing, two different names.
The covalent bond; covalent, covalence is an aspect of valence that I think comes
closer to the old-fashioned, rather vague concept of valence than do the other, more
precise aspects of valence that we make use of at the present time.
The theory of structural chemistry involving the idea of the covalent bond represented
by a line drawn between two symbols, the symbols for two atoms, this theory is one
of the greatest constructs of the human mind that has ever been formulated, perhaps
the greatest of all. The theory of structure, of structural chemistry, has been,
was developed about a hundred years ago. It was in 1852 that Frankland suggested
that different elements have different combining powers, can combine with different
numbers of atoms of other elements. In 1858, Couper and Kekule invented the idea
of the chemical bond and discovered the quadrivalence of carbon. Then, there were,
there was great progress in chemistry with the aid of this relatively simple theory
that had been obtained by induction from the many thousands of facts of chemistry.
It was possible for chemists to make the extraordinary progress that has led to modern
technology and medicine. The structure theory assisted the imagination of man in
such a way that he was able to make discoveries that would not have been made otherwise.
I should like to start out by talking about the simple structure theory, the simple
idea of the chemical bond in the substances that we now consider to contain covalent
bonds. The...as an example, I may take methane, represented by this model, a carbon
atom with four hydrogen atoms attached. The right orbit, the formula C, H, H, H,
H. And of course, Couper and Kekule recognized that the carbon atoms forming four
bonds can also be attached to one another.
I have another model, here, representing the structure of ethane, the next hydrocarbon
in the aliphatic series, C2H6, two carbon atoms attached together, six hydrogen atoms. Each carbon atoms is quadrivalent,
forms four bonds, one with the other carbon atom and three with the three hydrogen
atoms that in the structural formula are shown connected to it by the valence bonds.
As a more complicated example, I may show ethanol, ethyl alcohol. Here we have the
two carbon atoms bonded together, three hydrogen atoms, two here, a bond to the oxygen
atom, and then a bond to the hydrogen atom on oxygen. C2H5OH, ethanol.
I may mention that these ball and stick models are quite illuminating but they do
not give a really correct idea of the shape of the molecule. The ethanol molecule
does not have a thin portion that connects the carbon atom to the hydrogen atom.
Instead, the electron distribution in space is more satisfactorily represented by
this model, a space-filling model in which the atoms are drawn, are shown as spheres
with a radius that corresponds approximately to the contact, effective contact radius,
when the molecules are piled together in a liquid or a crystal. If there were another
molecule in solution, say in ethanol, for example this rather larger molecule, then
the molecules would roll over one another in such a way that they do, would not get
closer, much closer to one another than the distance corresponding to contact between
the spheres that represent the atoms in these models.