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.