Chemistry · CHEMICAL BONDING AND MOLECULAR STRUCTURE

In the resonance structures of ozone (\( \text{O}_3 \)) shown in Fig. 4.3 on the page, canonical forms I and II have distinct single and double bonds, while structure III represents the resonance hybrid. Which of the following statements correctly explains why structure III has identical \( \text{O}-\text{O} \) bond lengths of \( 128\text{ pm} \)?

1. The actual structure fluctuates rapidly between canonical forms I and II in a rapid dynamic equilibrium.
2. The two oxygen-oxygen bonds differ in energy, but spatial symmetry forces their physical bond lengths to appear identical.
3. The ozone molecule spends \( 50\% \) of its time as structure I and \( 50\% \) of its time as structure II.
4. Both canonical forms have no independent existence; the resonance hybrid represents the intermediate bond character with delocalised electron density.