JEE Main 2023 · 1 February · morning · Single correct · +4 / −1
Resonance in carbonate ion (CO₃²⁻) is
Which of the following is true?
- (a) It is possible to identify each structure individually by some physical or chemical method.
- (b) All these structures are in dynamic equilibrium with each other.
- (c) Each structure exists for equal amount of time.
- (d) CO₃²⁻ has a single structure i.e., resonance hybrid of the above three structures.
Answer and solution
Answer (d) CO₃²⁻ has a single structure i.e., resonance hybrid of the above three…
Solution
Chemistry ma’am · 3 steps
The three structures are only drawings on paper; the real carbonate ion is one hybrid, all the time.
- What the drawings mean. No single Lewis structure can show the true bonding in CO₃²⁻. So we draw three canonical structures that differ only in where the π electrons sit. None of them exists as a real species.
- What experiment shows. All three C–O bonds come out identical, at about 129 pm. That is between a C–O single bond (143 pm) and a C=O double bond (121 pm). Each bond has a bond order of 11/3, and each oxygen carries a charge of -2/3.
Option Claim Verdict (a) Each structure can be identified separately No: they do not exist, so nothing can detect them (b) Structures are in dynamic equilibrium No: equilibrium needs real species turning into each other (c) Each structure exists for equal time No: the ion does not flip between structures (d) One structure, the resonance hybrid Yes - So. CO₃²⁻ has a single real structure, the resonance hybrid of the three drawings. The answer is (d).
The idea to keep. Resonance is not a back-and-forth switch. The canonical forms are imaginary; only the hybrid is real, and it is more stable than any single drawing of it.
Answer from the NTA final answer key. Solution by PrepWiser.