Linus Pauling: There are many other substances for which a satisfactory structure cannot be written, a single satisfactory structure and
instead two or more, two or more structures must be written.
Ozone is an example. The ozone molecule consists of three oxygen atoms with about a hundred and twenty degree angle in this
region. If we try to assign a structure in which each of the stable orbitals, too many there, each of the stable orbitals
is used in forming a bond or for occupancy by unshared pairs, then we find that this is the best that we can do. Each oxygen
atom has now achieved the neon structure: two unshared pairs and two shared pairs, three unshared pairs and one shared pair,
one unshared pair and three shared pairs. But, this makes this oxygen-oxygen bond different from this, whereas it is known
that these oxygen-oxygen bonds are equivalent.
The solution to this difficulty is that there is another way of introducing the valence bonds in which the single bond and
the double bond have changed places. Ozone has a resonating structure. Each of these bonds can be described as being a hybrid
of a single bond and a double bond, a bond with about one and a half bond character.
This is a model, a small model, showing the correct packing dimensions of the atoms of sulfur dioxide. The two oxygen atoms
attach to sulfur, the bond angle again about a hundred twenty degrees. And, here again, there’s a double bond and a single
bond with resonance between the two structures, double bond, single bond and double bond, single bond.
The criticism has been made, rather often, of the theory of resonance, that it is artificial. It is said, for example, that
nobody has ever found a benzene, nobody has ever synthesized benzene that has one Kekule structure, and that accordingly,
one should not talk about the first Kekule structure and the second Kekule structure.
Now, the fact is that the theory of resonance is no more artificial than ordinary structure theory. It is true that nobody
has ever succeeded in bringing into the laboratory, a flask full, a beaker full of benzene with the first Kekule structure,
and another beaker full of benzene with the second Kekule structure. But in fact, nobody has ever succeeded in bringing into
the laboratory a beaker full of carbon-carbon single bonds or carbon-carbon double bonds or carbon-hydrogen bonds, and yet
we are happy to talk about the carbon-hydrogen single bonds, the carbon-carbon double bonds, as structural features of molecules,
of the ethylene molecule, the methane molecule, the ethane molecule.
In fact, of course, every molecule is something of its own. No two molecules are exactly alike. The carbon-carbon distance
in one molecule, the average carbon-carbon distance is a little different from the carbon-carbon distance in another molecule.
In isobutane, the carbon-carbon distance may differ by a few thousandths of an angstrom from that in ethane. Yet, the approximation
of the carbon-carbon distance to the standard value 1.54 is very good for many substances, the carbon-hydrogen distance often
has a standard value of about 1.08 angstrom.
And we have found it, chemists have found it, very useful to make, to talk about structures for molecules that involve the
idea of the carbon-carbon single bond and the carbon-carbon double bond, even though these are constructs of the intellect,
rather than a part of nature that can be completely isolated.
Well, in the same way, it is found very useful to talk about the resonance of the ozone molecule between this valence bond
structure and this valence bond structure or to speak of ozone as a hybrid that has a structure that can be represented by
two different valence bond structures.