Final answer:
The formate ion, HCO2-, has three possible resonance structures.
Step-by-step explanation:
The formate ion, HCO2- , has three possible resonance structures. In these resonance structures, the double bond between the carbon and one of the oxygen atoms can be formed from any of the three oxygen atoms. The actual arrangement of the electrons in the formate ion is the average of these three structures. Each resonance structure represents a way the electrons can be arranged while keeping the same geometry of the molecule.