The electron that form the second bond between the Carbon and Oxygen are delocalized over all the oxygens. This gives a more stable structure than if the double bond existed only between one oxygen and carbon.
Work Step by Step
Three resonance structures can be drawn for CO3^2- ion. Step by step explanation: After drawing a lewis structure, it can be seen that two oxygens have negative charges and only one oxygen has the octet complete. So, the electrons are delocalized so that the negatives are stabilized and that gives rise to a more stable structure. The actual bond that exists between each oxygen and carbon is not a double bond, but a 1/3 bond.