The Carbonate Ion ($C{O}_3^{2-}$) exhibits resonance because the $\pi$ electrons of the $C=O$ double bond can be delocalized over the three oxygen atoms.

• In each canonical structure, one oxygen is double-bonded to carbon (formal charge 0) and the other two are single-bonded (formal charge $-1$ each).
• There are 3 equivalent resonance structures.
• The actual structure (resonance hybrid) shows that all three $C-O$ bonds are of equal length, intermediate between a single and a double bond.