Explain the important aspects of resonance with reference to the ${\rm{CO}}_3^{2 - }$ ? ion.

Experimental bond length : The experimentally determined both carbon to oxygen bond length in $\mathrm{CO}_{2}$ is $115 \mathrm{pm}$.

The length of a normal carbon to oxygen double bond $(C=O)$ and carbon to oxygen triple bond $(\mathrm{C} \equiv \mathrm{O})$ are $121 \mathrm{pm}$ and $110 \mathrm{pm}$ respectively.

The $\mathrm{C}-\mathrm{O}$ bond lengths in $\mathrm{CO}_{2}(115 \mathrm{pm})$ lie between the values for $\mathrm{C}=\mathrm{O}$ and $\mathrm{C}=\mathrm{O}$.

Obxiouly, a single Lewis structure cannot depict this position and it becomes necessary to write more than one Lewis structures and to consider that the structure of $\mathrm{CO}_{2}$ is best described as a hybrid of the canonical or resonance forms $1$ , $II$ and $III.$

The hybrid resonance $(IV)$ shows the correct bond length.

$\ddot{\underline{\mathrm{O}}}=\mathrm{C}=\ddot{\mathrm{O}} \longleftrightarrow: \stackrel{+}{\mathrm{O}} \equiv \mathrm{C}-\ddot{\mathrm{O}}^{-} \leftrightarrow \stackrel{-}{\mathrm{O}}-\mathrm{C} \equiv \stackrel{+}{\mathrm{O}}: \equiv[\mathrm{O}=\mathrm{C}=\mathrm{O}]$

OR Resonance structure by Lewis represent are as follow.

$\ddot{0}:: \mathrm{C}:: \ddot{\mathrm{O}}: \leftrightarrow: \stackrel{+}{\mathrm{O}}:: \mathrm{C}: \ddot{\underline{0}}^{-} \leftrightarrow: \ddot{0}^{-} \mathrm{C}:: \mathrm{O}^{+}$