Resonance And Mesomeric Effect

Resonance describes a real molecule as a weighted blend of several canonical structures, and the rules for judging their relative importance follow from energetic stability. A canonical form contributes more when it has the maximum number of covalent bonds and when its atoms, especially second-period elements, satisfy the octet rule, because such a structure is lower in energy. Maximum separation of unlike charges is unfavourable since charge separation costs energy, so that option describes a minor contributor, not a major one. Placing negative charge on the least electronegative atom is destabilising; negative charge prefers the more electronegative atom, so that option is wrong. Having the fewest covalent bonds means more broken or dative interactions and higher energy, making it a poor contributor rather than a dominant one. Thus the form with more bonds and complete octets dominates the hybrid, in line with the NCERT guidelines on judging resonance contributors, which also state that a contributor should keep the same atomic positions and differ only in electron placement. Applying these rules lets a student rank canonical forms and predict where charge density actually accumulates in the real molecule, a skill tested repeatedly in JEE questions on stability and reactivity. Plausibility check: lower-energy, well-bonded, octet-complete structures resemble the true hybrid most closely, so they must contribute most, while highly charge-separated forms remain only minor contributors.