(N/A) Resonance Structures: Different Lewis structures of a single molecule or ion are known as resonance structures. According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, more than one Lewis structure (resonance structures) is used to describe it.
Resonance structures of ozone $(O_3)$: To draw resonance structures, the positions of the atomic nuclei do not change. In every resonance structure, the positions of bonding and non-bonding electrons change, but the overall Lewis structure does not appear different. The different resonance structures are represented by a double-headed arrow. All resonance structures have similar energy.
Two resonance structures of the $O_3$ molecule are $(I)$ and $(II)$, and $(III)$ is the resonance hybrid.
Limitations of resonance: No single structure is the correct structure. The $O-O$ single bond length is $148 \ pm$ and the $O=O$ double bond length is $121 \ pm$. Thus, experimentally, $O_3$ does not contain distinct single and double bonds.
$\therefore$ The correct structure of $O_3$ is not just $(I)$ or $(II)$.
Resonance: The correct structure is the resonance hybrid. Experimentally determined oxygen-oxygen $(O-O)$ bond lengths in the $O_3$ molecule are identical at $128 \ pm$. So, $(III)$ is the real or accurate structure of $O_3$ in which single and double bonds are not static. Resonance structures are necessary because they provide a more accurate representation than a single Lewis structure.
Requirement: A single assumed structure cannot correctly predict bond length and bond energy.
Advantage: It provides an estimation of the correct molecular structure.