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

And the Lewis structure is $\left(O =\right) C {\left(- {O}^{-}\right)}_{2}$.
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 $C {O}_{2}$, $C {O}_{3}^{2 -}$, ""^(-):C-=O^(+), and ${R}_{3} C - O H$ for reference.