Question

Explain the important aspects of resonance with reference to the \(CO_3^{2-}\) ion.

Answer 1

Resonance:

Resonance is a phenomenon in which a molecule or ion cannot be represented adequately by a single Lewis structure. Instead, it is described by two or more canonical structures that differ only in the arrangement of electrons. The actual structure is a resonance hybrid of all these structures.

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Resonance in carbonate ion (CO₃²⁻):

The carbonate ion cannot be represented by a single Lewis structure because the double bond between carbon and oxygen can be placed with any one of the three oxygen atoms.

Thus, CO₃²⁻ has three equivalent resonance structures. In each structure:

• One C=O double bond is present. 
• Two C–O single bonds are present. 
• The negative charge is shown on the singly bonded oxygen atoms.

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Important aspects of resonance illustrated by CO₃²⁻:

1. Delocalisation of electrons
The π electrons are delocalised over all three C–O bonds instead of being confined to one bond.

2. Equivalent bond lengths
All three C–O bonds in the carbonate ion are of equal length, intermediate between a single and a double bond.

3. Greater stability
The resonance hybrid is more stable than any individual canonical structure due to delocalisation of electrons.

4. Actual structure is a hybrid
The real structure of CO₃²⁻ does not oscillate between structures. It is a stable hybrid with equal contribution from all resonance forms.

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Conclusion:

Thus, the carbonate ion clearly demonstrates the concept of resonance, showing electron delocalisation, equal bond lengths, and enhanced stability of the resonance hybrid.