Carbonate Guide

The carbonate ion is the simplest oxocarbon anion. It consists of one carbon atom surrounded by three identical oxygen atoms, in a trigonal planar arrangement, with D3hmolecular symmetry. It has a molecular mass of 60.01 daltons and carries a negative two formal charge. It is the conjugate base of the hydrogen carbonate (bicarbonate) ion, HCO3−, which is the conjugate base of H2CO3, carbonic acid.

The Lewis structure of the carbonate ion has two (long) single bonds to negative oxygen atoms, and one short double bond to a neutral oxygen

This structure is incompatible with the observed symmetry of the ion, which implies that the three bonds are equally long and that the three oxygen atoms are equivalent. As in the case of the isoelectronic nitrate ion, the symmetry can be achieved by a resonance between three structures:

This resonance can be summarized by a model with fractional bonds and delocalized charges:...