Q1(a). Resonance energy; resonance in the carbonate ion

1. Resonance energy: the difference between the energy of the most stable contributing (canonical) structure and the energy of the actual molecule. It is found as (calculated - experimental) enthalpy. Benzene: hydrogenation of three isolated C=C would release 359 kJ mol⁻¹; benzene releases 208 kJ mol⁻¹, so its resonance energy is 151 kJ mol⁻¹ (36 kcal mol⁻¹).
2. Conditions for resonance: (i) the positions of the nuclei are the same in every structure; only electrons are shifted. (ii) Only π electrons and lone pairs move. (iii) The number of paired and unpaired electrons is the same. (iv) The atoms taking part lie in one plane, so their p orbitals overlap. (v) The structures have comparable energy; the more nearly equivalent they are, the larger the resonance energy.
3. CO₃²-, the structures: C is sp²; it forms three σ bonds in a plane at 120°. The fourth (p) electron of C forms one π bond, which can be placed on any of the three O atoms. This gives three equivalent structures of equal energy.
4. The hybrid: the real ion is none of them. The π pair is spread over a four-centre π system (C + 3 O). Each C–O bond has bond order (1+1+2)/3=4/3; each O carries -2/3.
5. Evidence: all three C–O bonds are of equal length, between a C–O single and a C=O double bond. The ion is trigonal planar (D₃ₕ), all O–C–O angles 120°.
6. Stability: three equal structures give a large resonance energy; the negative charge is spread over three O atoms, so CO₃²- is more stable than any one structure suggests.

Application. the same pattern holds for the isoelectronic NO₃⁻ and BO₃³-; delocalisation of charge in RCOO⁻ (two equivalent structures) explains why carboxylic acids are far stronger acids than alcohols.