Question
For resonance forms of a molecule or ion:
A. One always corresponds to the observed structure.
B. The observed structure keeps alternating between different resonance forms.
C. The observed structure is an average of the resonance forms.
D. The same atoms need not be bonded to each other in all resonance forms.
Answer
C. The observed structure is an average of the resonance forms.
The real molecule or ion has one continuous electronic structure called a resonance hybrid. Individual resonance forms are drawing conventions that place π electrons, lone pairs, and formal charges in different allowed positions. No single form is the complete observed structure.
Why the other choices are incorrect
- A: One resonance form does not become the uniquely observed structure. Each valid contributor describes part of the hybrid.
- B: The species does not alternate between drawings. Electron density is delocalized in one structure.
- D: Valid resonance forms keep the same atoms in the same positions with the same sigma-bond connectivity. Only electron placement changes.
“Average” should be read as a structural composite. Equivalent contributors make equal contributions; nonequivalent contributors can contribute unequally, with the more stable form influencing the hybrid more strongly.
Evidence boundary
“Average” is the wording used by this multiple-choice version for the resonance hybrid. It is not a time average and does not imply rapid switching. The page follows the Marshall four-option question and does not mix in the additional distractor found in other versions.
Sources
These references support the concepts and methods used in the explanation above.