Final answer:
The Lewis structure for CO₃²⁻ includes one double bond between carbon and one of the oxygen atoms, while the other two oxygen atoms have single bonds with carbon. This double bond is necessary to satisfy the octet rule for the central carbon atom.
Step-by-step explanation:
The Lewis structure for the carbonate ion, CO₃²⁻, involves a total of 24 valence electrons, consisting of 22 from the oxygen atoms and carbon atom, plus the additional two electrons from the ion's charge. The central carbon atom is surrounded by three oxygen atoms. Each oxygen atom initially forms a single bond with carbon, using up six electrons (two for each bond). To give the carbon atom a full octet, one of the single bonds with oxygen must be converted into a double bond, which is a sharing of two pairs (four) of valence electrons between carbon and oxygen.
While only one double bond is shown in a single Lewis structure, the true structure of CO₃²⁻ is a resonance hybrid of three equivalent structures. As a result, all three C-O bonds are, in reality, identical, meaning that each bond has partial double bond character. Nevertheless, when asked to draw a Lewis structure, we depict one of the possible resonance forms with one double bond and two single bonds, acknowledging that it is a simplification of the true bonding situation.