It is often observed that a single Lewis structure is inadequate for the representation of a molecule in conformity with its experimentally determined parameters. For example, the ozone, O3 molecule can be equally represented by the structures I and II shown below: In both structures we have a O–O single bond and a O=O double bond. The normal O–O and O=O bond lengths are 148 pm and 121 pm respectively. Experimentally determined oxygen-oxygen bond lengths in the O3 molecule are same (128 pm). Thus the oxygen-oxygen bonds in the O3 molecule are intermediate between a double and a single bond. Obviously, this cannot be represented by either of the two Lewis structures shown above.
-- NCERT Class 11 Chemistry, Ch. 4, p. 109Resonance
8 MCQs9-step worked example
Source: NCERT Chemical Bonding and Molecular StructurePYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 10 Oct 2026
Try this first
How is resonance between canonical structures represented?
- A.By two opposite half-arrows, as for a chemical equilibrium
- B.By a single-headed arrow
- C.By a double headed arrow
- D.By a dotted line joining the structures
Tap to see the answer
Answer: C. C is correct. NCERT: “Resonance is represented by a double headed arrow.” (NCERT Class 11 Chemistry, Chapter 4, page 109).
A is wrong: A is wrong because equilibrium arrows would suggest the molecule interconverts between the structures. NCERT states there is no such equilibrium between canonical forms (trap: resonance treated as equilibrium).
B is wrong: B is wrong because a single-headed arrow shows a reaction direction or electron movement, not the relation between canonical structures.
D is wrong: D is wrong because a dotted line is not the notation NCERT gives for resonance; the double headed arrow is.
Resonance, explained for NEET
Does ozone flip back and forth between two structures, spending half its time in each? No, and that picture is the trap this topic is built on. NCERT lists it as a misconception, and a statement of this kind is an easy NEET distractor.
Why a single Lewis structure fails. The two Lewis structures of O₃ each show one O–O single bond and one O=O double bond. A normal O–O bond is 148 pm and a normal O=O bond is 121 pm. The measured O–O bond lengths in O₃ are the same, 128 pm, so the bonds are intermediate between single and double, and neither drawing shows that (NCERT Class 11 Chemistry, Chapter 4, page 109).
The idea of resonance. When one Lewis structure cannot describe a molecule accurately, we draw several structures with similar energy and the same positions of nuclei. These are the canonical (resonance) structures. The molecule is their hybrid. Resonance is written with a double headed arrow (NCERT Class 11 Chemistry, Chapter 4, page 109).
Same pattern elsewhere. All C–O bonds in the carbonate ion CO₃²⁻ are equivalent, so it is a hybrid of three canonical forms. In CO₂ the C–O length is 115 pm, between C=O (121 pm) and C≡O (110 pm), so it is also described as a hybrid of three canonical forms (NCERT Class 11 Chemistry, Chapter 4, page 110).
What resonance does. The hybrid has lower energy than any single canonical structure, so resonance stabilises the molecule and averages the bond characteristics (NCERT Class 11 Chemistry, Chapter 4, page 110).
Watch-outs from NCERT (page 110).
- Canonical forms have no real existence.
- The molecule is not in one form for a fraction of the time and another form for the rest.
- There is no equilibrium between canonical forms, unlike the keto and enol forms in tautomerism.
- The molecule has one structure, the hybrid, which no single Lewis structure can depict.
How do you solve a Resonance question? A worked example
Pattern: Use measured bond lengths to decide whether a single Lewis structure is enough, a PYQ-style question type for this topic.
- 1
Given
Observed O–O bond lengths in O₃: 128 pm and 128 pm (the two bonds are the same). Normal O–O: 148 pm. Normal O=O: 121 pm.
- 2
Required
Whether a single Lewis structure represents O₃, and the correct description of the molecule.
- 3
Concept
Any single Lewis structure of O₃ has one O–O single bond and one O=O double bond, which would have different lengths. When measured data cannot be matched by one structure, resonance is used: the molecule is a hybrid of canonical structures (NCERT Class 11 Chemistry, Chapter 4, page 109).
- 4
Formula
No formula is needed. Compare the observed lengths with the normal single-bond and double-bond lengths.
- 5
Substitution
Compare 128 pm with the range 121 pm to 148 pm. Compare the two observed bonds with each other.
- 6
Calculation
121 pm < 128 pm < 148 pm, so each bond is intermediate between a double and a single bond. The two bonds are equal (128 pm = 128 pm), but a single Lewis structure would give 148 pm for one bond and 121 pm for the other. The numbers 1 and 2 (counting bonds and structures) are exact counts and do not affect significant figures.
- 7
Final answer
No single Lewis structure fits. O₃ is described as a resonance hybrid of two canonical structures. The hybrid is more stable than either canonical form.
- 8
Common trap
Reading the two canonical structures as real forms that O₃ switches between, or as being in equilibrium. NCERT states that canonical forms have no real existence and that there is no such equilibrium (page 110).
- 9
Similar NEET-style question
The measured C–O bond length in CO₂ is 115 pm; a normal C=O is 121 pm and a normal C≡O is 110 pm. What does this show? (Answer: 110 pm < 115 pm < 121 pm, so the bond lies between C≡O and C=O; one Lewis structure cannot depict this, and CO₂ is a hybrid of canonical forms I, II and III.)
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Can you answer these Resonance MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
How is resonance between canonical structures represented?
Show answer and why every option is right or wrong
Answer: C. C is correct. NCERT: “Resonance is represented by a double headed arrow.” (NCERT Class 11 Chemistry, Chapter 4, page 109).
Why A is wrong: A is wrong because equilibrium arrows would suggest the molecule interconverts between the structures. NCERT states there is no such equilibrium between canonical forms (trap: resonance treated as equilibrium).
Why B is wrong: B is wrong because a single-headed arrow shows a reaction direction or electron movement, not the relation between canonical structures.
Why D is wrong: D is wrong because a dotted line is not the notation NCERT gives for resonance; the double headed arrow is.
MCQ 2Easy RecallPractice
What do experiments give for the oxygen-oxygen bond lengths in the O₃ molecule?
Show answer and why every option is right or wrong
Answer: A. A is correct. NCERT: “Experimentally determined oxygen-oxygen bond lengths in the O3 molecule are same (128 pm).” (NCERT Class 11 Chemistry, Chapter 4, page 109).
Why B is wrong: B is wrong because 148 pm and 121 pm are the normal O–O and O=O lengths that either single Lewis structure of O₃ would predict. The measured bonds are equal (trap: reading the drawn structure as the real one).
Why C is wrong: C is wrong because 121 pm is the length of a normal O=O double bond. The O₃ bonds are longer, 128 pm.
Why D is wrong: D is wrong because 148 pm is the length of a normal O–O single bond. The O₃ bonds are shorter, 128 pm.
MCQ 3Easy RecallPractice
For which situation was the concept of resonance introduced?
Show answer and why every option is right or wrong
Answer: D. D is correct. NCERT: “The concept of resonance was introduced to deal with the type of difficulty experienced in the depiction of accurate structures of molecules like O3.” (NCERT Class 11 Chemistry, Chapter 4, page 109).
Why A is wrong: A is wrong because the presence or absence of lone pairs is not the reason; O₃ itself has lone pairs on its atoms.
Why B is wrong: B is wrong because resonance is not about unknown formulas. O₃ has a known formula; the difficulty is drawing its structure.
Why C is wrong: C is wrong because ionic lattices are a separate topic. Resonance deals with molecules and ions whose bonds one Lewis structure cannot represent.
MCQ 4Direct ApplicationPractice
The measured carbon-oxygen bond length in CO₂ is 115 pm, while a normal C=O bond is 121 pm and a normal C≡O bond is 110 pm. What follows?
Show answer and why every option is right or wrong
Answer: B. B is correct, since 110 pm < 115 pm < 121 pm. Based on NCERT Class 11 Chemistry, Chapter 4, page 110.
Why A is wrong: A is wrong because a normal C=O length is 121 pm, but CO₂ measures 115 pm. The bond is shorter than a typical double bond.
Why C is wrong: C is wrong because it describes one fixed structure, which a single Lewis structure cannot justify. The measured length is intermediate, not that of a single or triple bond.
Why D is wrong: D is wrong because the data give one C–O length, 115 pm, for the molecule, not two different lengths.
MCQ 5Direct ApplicationPractice
Experiments show all three carbon-oxygen bonds in the carbonate ion CO₃²⁻ are equivalent. Which description fits?
Show answer and why every option is right or wrong
Answer: A. A is correct. NCERT: “Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II, and III shown below.” (NCERT Class 11 Chemistry, Chapter 4, page 110).
Why B is wrong: B is wrong because a fixed structure with two single bonds and one double bond represents unequal bonds, which is why NCERT calls it inadequate (trap: trusting a single drawn structure).
Why C is wrong: C is wrong because there is no equilibrium between canonical forms. The ion has one structure, the hybrid.
Why D is wrong: D is wrong because there is only one carbonate ion, with all three C–O bonds equivalent; nothing in the data suggests a mixture.
MCQ 6Direct ApplicationPractice
How does the energy of the O₃ resonance hybrid compare with its canonical forms?
Show answer and why every option is right or wrong
Answer: D. D is correct. The hybrid is more stable than any single canonical structure, so resonance stabilises the molecule. Based on NCERT Class 11 Chemistry, Chapter 4, page 110.
Why A is wrong: A is wrong because NCERT says the energy of the hybrid is less than the energy of any single canonical structure, not merely their average. Bond characteristics are averaged, not energy (trap: mixing the two statements).
Why B is wrong: B is wrong because a higher energy would make resonance destabilising. The hybrid is lower in energy.
Why C is wrong: C is wrong because the hybrid is lower than even the more stable canonical form, so it is not equal to it.
MCQ 7CalculationPractice
Consider these statements about O₃. (i) It spends part of the time in one canonical form and part in the other. (ii) Its two O–O bonds are equal in length. (iii) The hybrid has lower energy than either canonical form. (iv) The canonical forms are real structures that exist. Which statements are correct?
Show answer and why every option is right or wrong
Answer: B. B is correct. The bonds are equal (128 pm) and the hybrid is lower in energy. NCERT: “The cannonical forms have no real existence.” (NCERT Class 11 Chemistry, Chapter 4, page 110). That rules out (iv), and the same page rules out (i).
Why A is wrong: A is wrong because (i) is the misconception that the molecule exists for fractions of time in each form. The molecule has one structure, the hybrid (trap: flipping between forms).
Why C is wrong: C is wrong because (iv) is false. Canonical forms are only a way of describing the hybrid and have no real existence.
Why D is wrong: D is wrong because both (i) and (iv) are listed misconceptions; neither is correct.
MCQ 8CalculationPractice
A student writes: "In CO₃²⁻ the three canonical forms are in equilibrium, just like the keto and enol forms of a tautomeric compound." What is wrong with this?
Show answer and why every option is right or wrong
Answer: C. C is correct. Tautomers are different compounds that interconvert; canonical forms are not. NCERT: “There is no such equilibrium between the cannonical forms as we have between tautomeric forms (keto and enol) in tautomerism.” (NCERT Class 11 Chemistry, Chapter 4, page 110).
Why A is wrong: A is wrong because it repeats the student's error. Resonance describes one structure, the hybrid, not a shifting mixture of forms (trap: resonance treated as equilibrium).
Why B is wrong: B is wrong because canonical forms have no real existence, so none of them can be called dominant in the molecule.
Why D is wrong: D is wrong because carbonate is described by three canonical forms, I, II and III, and even so there is no equilibrium between them.
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What to remember before solving Resonance questions
7 NCERT lines
The concept of resonance was introduced to deal with the type of difficulty experienced in the depiction of accurate structures of molecules like O3. According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately. Thus for O3, the two structures shown above constitute the canonical structures or resonance structures and their hybrid i.e., the III structure represents the structure of O3 more accurately. This is also called resonance hybrid. Resonance is represented by a double headed arrow.
-- NCERT Class 11 Chemistry, Ch. 4, p. 109The single Lewis structure based on the presence of two single bonds and one double bond between carbon and oxygen atoms is inadequate to represent the molecule accurately as it represents unequal bonds. According to the experimental findings, all carbon to oxygen bonds in CO3 2– are equivalent. Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II, and III shown below.
-- NCERT Class 11 Chemistry, Ch. 4, p. 110Key Fact
CO2: C-O 115 pm lies between C=O 121 pm and C-triple-O 110 pm; hybrid of three canonical forms
The experimentally determined carbon to oxygen bond length in CO 2 is 115 pm. The lengths of a normal carbon to oxygen double bond (C=O) and carbon to oxygen triple bond (C≡O) are 121 pm and 110 pm respectively. The carbon-oxygen bond lengths in CO2 (115 pm) lie between the values for C=O and C≡O. Obviously, a single Lewis structure cannot depict this position and it becomes necessary to write more than one Lewis structures and to consider that the structure of CO2 is best described as a hybrid of the canonical or resonance forms I, II and III.
-- NCERT Class 11 Chemistry, Ch. 4, p. 110Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single cannonical structure; and, • R e s o n a n c e a v e r a g e s t h e b o n d characteristics as a whole. Thus the energy of the O3 resonance hybrid is lower than either of the two cannonical froms I and II (Fig. 4.3).
-- NCERT Class 11 Chemistry, Ch. 4, p. 110Many misconceptions are associated with resonance and the same need to be dispelled. You should remember that : • The cannonical forms have no real existence. • The molecule does not exist for a certain fraction of time in one cannonical form and for other fractions of time in other cannonical forms. • There is no such equilibrium between the cannonical forms as we have between tautomeric forms (keto and enol) in tautomerism. • The molecule as such has a single structure which is the resonance hybrid of the cannonical forms and which cannot as such be depicted by a single Lewis structure.
-- NCERT Class 11 Chemistry, Ch. 4, p. 110For O3, the two Lewis structures are its canonical/resonance forms, with a third (hybrid) structure representing O3 accurately. For the carbonate ion CO3^2-, since all C-O bonds are equivalent, it is best described as a resonance hybrid of three canonical forms I, II and III.
-- NCERT Class 11 Chemistry, Ch. 4, p. 110Resonance: NEET previous year questions (PYQs) with answers
1 question from NEET 2024, answers verified against NTA official keys
NEET 2024Revised key
1Three resonance structures can be drawn for ozone
2BF₃ has non-zero dipole moment
3Dipole moment of NF₃ is greater than that of NH₃
4Three canonical forms can be drawn for CO₃²⁻ ion
NTA Answer: Option 4(revised_final)
Why the other options are wrong
- Option 1: Ozone has two canonical forms, not three: for O3, the two structures shown above constitute the canonical structures or resonance structures, and structure III is their hybrid.
- Option 2: BF3 has zero dipole moment: the three bond moments give a net sum of zero as the resultant of any two is equal and opposite to the third.
- Option 3: It is the other way round: the dipole moment of NH3 (4.90 × 10–30 C m) is greater than that of NF3 (0.8 × 10–30 C m).
All 18 past-paper questions from Chemical Bonding and Molecular Structure →
More in Chemical Bonding and Molecular Structure: 4 exam traps and mistakes · 2 formulas · 3 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 4, p.109 | Class 11 Chemistry Chapter 4, p.110
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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