Bond Lengths
Comparing the two contributing structures of benzene, all single and double bonds are interchanged. Bond lengths can be measured, for example using X-ray diffraction. The average length of a C-C single bond is 154 pm; that of a C=C double bond is 133 pm. In localized cyclohexatriene, the carbon-carbon bonds should be alternating 154 and 133 pm. Instead, all carbon-carbon bonds in benzene are found to be about 139 pm, a bond length intermediate between single and double bond. This mixed single and double bond (or triple bond) character is typical for all molecules in which bonds have a different bond order in different contributing structures.
Read more about this topic: Resonance (chemistry)
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