Q.Write the resonance structures of CO3^2– and HCO3–.

Step 1 - Carbonate ion, $\text{CO}_3^{2-}$

Carbon is bonded to three oxygen atoms. A single Lewis structure would show one C=O double bond and two C-O single bonds (each carrying a negative charge), predicting unequal bond lengths. But three such structures can be drawn, differing only in which oxygen carries the double bond:

These three equivalent resonance structures are in resonance (not real, separate species); the true structure is a hybrid where all three C-O bonds are identical, each of intermediate bond order (about $1\tfrac{1}{3}$), consistent with the experimentally equal C-O bond lengths (about 1.31 A) and the trigonal planar, $sp^2$ shape of $\text{CO}_3^{2-}$.

Step 2 - Bicarbonate ion, $\text{HCO}_3^-$

Here one oxygen carries the H (as -OH, a fixed, non-resonating single bond), while carbon is doubly bonded to one of the remaining two oxygens and singly bonded (with the negative charge) to the other. Two resonance structures can be drawn by swapping which of the two non-protonated oxygens bears the double bond: …

Draw the Lewis structure with formal charges, identify which bonds/atoms can exchange the double-bond position without moving any atoms, and draw each equivalent resonance form - for HCO3- keep the C-OH bond f …

• Drawing only one Lewis structure for CO3^2- and missing that resonance requires all three equivalent forms.
• Letting the double bond migrate onto the protonated oxygen in HCO3- (chemically wrong - that oxygen's bonds are already fixed by the H). …