Resonance is an important concept in IB Chemistry Topic 4 (Bonding) and Topic 10 (Organic Chemistry). It helps explain why some molecules cannot be described by a single Lewis structure. Instead, they exist as a hybrid of multiple contributing structures. Resonance is essential for understanding molecular stability, bond order, delocalized electrons, and the behavior of many organic and inorganic ions.
What Is Resonance?
Resonance occurs when more than one valid Lewis structure can be drawn for a molecule, and the real molecule is a hybrid of all these structures.
This happens when:
- Electrons can be placed in more than one arrangement
- Double or lone-pair electrons can be delocalized
- Multiple bonding positions are possible
The resonance structures themselves are not real.
The resonance hybrid—a blend of all structures—is the true representation.
Why Resonance Happens
Resonance occurs because some molecules contain electrons that are delocalized, meaning they are shared across more than two atoms rather than belonging to a single bond.
Delocalization:
- Lowers energy
- Increases stability
- Evens out charge distribution
When electron pairs can be placed in multiple locations without breaking rules, resonance is required.
Key Features of Resonance Structures
- Placement of atoms stays the same.
Only electrons move—never atoms. - Only π electrons and lone pairs shift, not σ-bonds.
- All contributing structures must be valid Lewis structures.
- The actual molecule is a resonance hybrid, not flipping between structures.
- Bond lengths become intermediate, not fully single or double.
Examples of Resonance in IB Chemistry
1. Ozone (O₃)
Two resonance structures show the double bond alternating positions.
Actual molecule:
- Both O–O bond lengths identical
- Bond order = 1.5
- Delocalized π electrons spread across three atoms
2. Nitrate ion (NO₃⁻)
Three equal resonance structures.
Features:
- All N–O bonds identical
- Bond order = 4/3
- Negative charge spreads across all oxygens
This delocalization increases stability significantly.
3. Benzene (C₆H₆)
Shows six-membered ring with alternating double bonds.
But in reality:
- All C–C bonds are identical
- Bond order = 1.5
- Electrons delocalized over the entire ring
This is crucial in organic chemistry and aromaticity.
4. Carboxylate ions (RCOO⁻)
Two resonance structures with negative charges on different oxygens.
Features:
- Both C–O bonds equal in length
- Charge evenly distributed
- Ion becomes far more stable
This stability explains why carboxylic acids are acidic.
Resonance and Bond Order
Bond order for resonance hybrids is calculated as:
Bond order = (total number of bonding pairs) ÷ (number of bond positions)
Examples:
- O₃ → 3 bonding pairs / 2 positions = 1.5
- NO₃⁻ → 4 bonding pairs / 3 positions ≈ 1.33
- Benzene → 9 bonding pairs / 6 positions = 1.5
Bond order affects:
- Bond strength
- Bond length
- Reactivity
Why Resonance Increases Stability
Resonance lowers energy because:
- Electron density spreads out
- Charge is distributed evenly
- No single atom holds too much charge
- Delocalized systems are harder to break
This stability is why nitrate, sulfate, benzene, and carboxylates are common and chemically important.
How to Identify Resonance in IB Exams
Look for:
- Lone pairs next to double bonds
- Atoms with multiple ways to place double bonds
- Symmetrical polyatomic ions
- Aromatic rings
- Charged groups that can shift location
Common exam molecules include NO₃⁻, CO₃²⁻, SO₄²⁻, phenoxide ion, benzene, and amide groups.
Common IB Misunderstandings
“The molecule flips between resonance structures.”
Incorrect—the molecule is a hybrid, not switching.
“Resonance structures have equal stability.”
Not always—some contribute more to the hybrid than others.
“Only rings show resonance.”
Many linear ions have resonance too.
“Resonance is the same as isomerism.”
No—resonance structures are not isomers, and atoms never move.
FAQs
Why do resonance hybrids have intermediate bond lengths?
Because electrons are shared over several bonds, making all bonds partially double.
Can a molecule have more than two resonance structures?
Yes—some ions like nitrate and benzene have three or more.
How do I show resonance in drawings?
Use double-headed arrows between structures.
Conclusion
Resonance describes molecules that cannot be represented by one Lewis structure. Instead, they are hybrids of multiple contributing forms, with delocalized electrons and enhanced stability. Resonance is essential in IB Chemistry for understanding bond order, aromaticity, polyatomic ions, and organic molecule behavior.
