Question

How many resonance structures are required in the electron-dot structure of `CO_3^(2-)`?

Answer 1

Well, we would usually draw THREE resonance isomers......

Explanation:

And the Lewis structure is `(O=)C(-O^(-))_2`.

Of course, in the actual molecule/ion, ALL of the `C-O` bonds are equivalent. You should look up the `C-O` bond lengths in `CO_2`, `CO_3^(2-)`, `""^(-):C-=O^(+)`, and `R_3C-OH` for reference.