We are back at the same dilemma encountered in Chapter 4 with the benzene molecule. The real carbonate ion cannot be described by simple single and double electron-pair bonds. The two negative charges that are left behind when carbonic acid dissociates actually are delocalized, or spread over the entire carbonate ion.

Each C-O bond is intermediate between a single and a double bond. The true ion can be thought of as a combination of all of the trial structures drawn on the right, but corresponds to no one of them. We can show the ion either with partial double bonds, or with a cloud of delocalized electrons, as at the end of the animation.

X-ray crystal structure analyses of carbonate compounds reveal that each
C-O bond is 1.36 Å long.

Single C-O bonds in other compounds are 1.43 Å long, and double C=O bonds are 1.23 Å. From these values alone, one would estimate that each carbonate bond has approximately one third of a double-bond character, as if the actual ion were the average of the three models with two single bonds and one double bond shown on the next page.