Resonance in Ozone (O3)

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10-5-2019

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Fig. 1.2: another valid Lewis dot structure of ozone

So which of the above two structures is the correct one that accurately describes ozone?

Neither, as both bonds are between oxygen and oxygen, and so should be of the same length. That is, if you think about it, a double bond should be shorter than a single bond. There are two facets of logic to this. First, the double bond is stronger, because if you break the double bond, you end up with the single bond. Second, the stronger the bond attraction, the closer the nuclei, and so the stronger double bond will be shorter than the weaker single bond. But the bonds are between identical atoms with identical connectivity, and so they should be the same length. So we introduce the concept of Resonance, and the above two structures are actually resonance structures, with the real structure being a mixture of the two (you could say that bond is not single or double, but 1.5).

Fig. 1.3: The two resonance structures for ozone. Note the two headed arrow represents resonance structures, and neither of these is the actual molecule, with the real molecule being the mixture of these two structures.

In reality, what we will learn in chapter 9 is that the second bond of the double bond, which is called a π