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There are a number of compounds and polyatomic ions that cannot be written using one single structure. This was known even back to the early beginnings of structural chemistry in the mid-1850s. These substances must be described in terms of "intermediate" structures, possessing non-integral bonds such as one and one-half bonds or one and one-third bonds.
Beginning in the 1920s, the first modern attempts to explain these structures started. Starting around 1930, Linus Pauling developed what today is called "resonance theory," the currently accepted way to explain the bonding in these substances. The last portion of the first sentence of "The Nature of the Chemical Bond" reads:
". . . we are now ready to begin the discussion of the structure of molecules to which a single valence-bond formula cannot be assigned."
The acetate anion is an example (Replace R with -CH3 to get acetate):
Notice how the double bond can be shown attached to either oxygen. Both structures obey all the rules and there is NOTHING to rule out one structure in favor of the other.
Here is another example, using the molecule NO2:
Once again, note that both structures are completely within the rules.
Just one more example. Let's try a compound that has the nitro-group attached, such as ClNO2 or FNO2. However, I've put the answer in a different file, so you could try and figure it out on your own, if you wanted to. In the answer, I used RNO2, where the R stands for Cl or F.
So. What is resonance? Resonance happens when more than one valid Lewis dot-diagram (or what Pauling calls a valence-bond structure) can be written for a molecule or ion. When this happens, the true structure is a blend of all the different possible structures.
Also, understand that the different structures contribute differently to the final structure. I will try and highlight this when I do CO and CO2.
Now we come to some tough parts of resonance. Go back and stare at the the two NO2 structures. Let your eye go back and forth between the two for a few seconds.
OK, here's the deal. Neither one of those two structures really does exist. The real molecule that exists in nature is a "resonance hybrid" between the two. The real molecule acts as if it had one and one-half bonds between each of the two structures. The two structures above are merely descriptive aids and, in fact, never exist.
"We might say . . . that the molecule cannot be satisfactorily represented by any single valence-bond structure and abandon the effort to correlate its structure and properties with those of other molecules. By using valence-bond structures as the basis for discussion, however, with the aid of the concept of resonance, we are able to account for the properties of the molecule in terms of those of other molecules in a straightforward and simple way. It is for this practical reason that we find it convenient to speak of the resonance of molecules among several electronic structures."
OK, so you understand that the Lewis structures for NO2, for example, don't really exist. However, there is a big misconception that some people (hey, lots of people) get. What they do is they think that the actual molecule "flips" between the two structures (or "cycles through 3 or more structures).
Like if you were to make a movie of, say, the right-side nitrogen-oxygen bond. First, it's single, then double. The it goes back to single, then double, single, double, .... You get the idea. Since double bonds are shorter than singles, the O would pump in and out relative to the N. And as the right-side moves in, the left-side moves out. Back and forth, in and out. Yadda yadda yadda.
NO NO NO NO NO and I say NO NO NO NO. Let me repeat that. NO NO NO. What I just described DOES NOT happen.
The NO2 shows two nitrogen-oxygen bonds that are stable and both consistent with being the equivalent of one and one-half bonds. The true molecule consists of only ONE arrangement of nitrogen, oxygen and electrons. NOT two.
We cannot write that structure becuase of the limitations of paper-and-pencil. What we do write might be called freeze-frames of the most extreme bondings in the molecule. Then we say the true molecule is some mixture of those freeze-frames.
Try and determine the resonance structures for these examples:
ozone - O3
carbon monoxide - CO
carbon dioxide - CO2
I'm going to leave benzene - C6H6 - for you to discover. It is discussed in many textbooks and most probably a search on the web will bring up some good hits. There are five resonance structures considered to be important. Two of them (called the Kekulé structures) are the most important, the lesser three are collectively named the Dewar structures. By the way, benzene is the only substance I know of that has resonance strucures named after people. I hope this lets you see that benzene was (and still is) a critically important compound in chemistry.
A few more, with no answers:
carbon disulfide - CS2
nitrous oxide - N2O (nitrogen is the central atom)
azide ion - N3¯
carbon suboxide - C3O2 (the oxygens are at the ends of a chain)
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