The carbonate ion is as hard to represent by a simple Lewis electron-dot model as the benzene molecule is.

One can draw many possible structures, and several of these are shown on the right. Three of these would suggest that carbon is double-bonded to one oxygen, and single-bonded to the two oxygens from which the protons split away. These structures differ only in the choice of the oxygen atom to receive the double bond.

None of these structures are correct, because x-ray studies of crystals of carbonate compounds show that all three C-O bonds in the carbonate ion are identical, and that the oxygens lie at the corners of an equilateral triangle around the carbon atom.

We can draw such a symmetrical model for the carbonate ion, but it is not very satisfactory because it places a negative charge on each oxygen atom and a positive charge on the carbon.