Resonance Structures
Explains how a single Lewis structure cannot describe the bonding in some molecules, requiring multiple contributing structures (resonance structures or canonical forms).
What is Resonance Structures?
Hypothetical Lewis structures that represent the delocalization of electrons within a molecule or ion, none of which accurately describe the true bonding alone.
Key formula / rule: Electron Movement Rule 1
Key points
- Define resonance and resonance structures.
- Identify molecules/ions that exhibit resonance.
- Draw all valid resonance structures for a given species.
- Determine the relative stability of different resonance structures.
Common exam trap
Moving atoms instead of just electrons.
Definitions
- Term
Resonance Structures (Canonical Forms)
- Meaning
Hypothetical Lewis structures that represent the delocalization of electrons within a molecule or ion, none of which accurately describe the true bonding alone.
- Term
Resonance Hybrid
- Meaning
The actual, true electronic structure of a molecule or ion that exhibits resonance, which is an average or blend of all contributing resonance structures. It is more stable than any single canonical form.
- Term
Delocalization
- Meaning
The spreading of electron density over a larger area in a molecule, typically involving π electrons or lone pairs, leading to increased stability.
Learning objectives
Define resonance and resonance structures.
Identify molecules/ions that exhibit resonance.
Draw all valid resonance structures for a given species.
Determine the relative stability of different resonance structures.
Explain the concept of resonance hybrid and its significance.
Relate resonance to molecular stability, bond lengths, and reactivity.
Formulae
- Name
Electron Movement Rule 1
- Note
Moves a lone pair to form a π bond, and pushes the existing π bond electrons to an adjacent atom as a lone pair.
- Expression
Lone pair → adjacent π-bond → adjacent atom
- Name
Electron Movement Rule 2
- Note
Involves moving π electrons to form a new π bond, pushing existing π bond electrons to an adjacent atom.
- Expression
π-bond → adjacent π-bond → adjacent atom
- Name
Electron Movement Rule 3
- Note
Moves π electrons to an adjacent atom, forming a lone pair.
- Expression
π-bond → adjacent atom
- Name
Formal Charge Calculation
- Note
Used to assess the stability of resonance structures; structures with minimal formal charges are more stable.
- Expression
FC = (Valence e-) - (Non-bonding e-) - (1/2 Bonding e-)
Prerequisites
Lewis structures and octet rule.
Understanding of σ (σ) and π (π) bonds.
Concept of formal charge calculation.
Basic understanding of electronegativity.
Knowledge of hybridization (sp, sp2, sp3).
Common mistakes
Moving atoms instead of just electrons.
Changing the number of valence electrons in different resonance structures.
Violating the octet rule for second-period elements (e.g., carbon, nitrogen, oxygen, fluorine).
Assuming resonance structures are rapidly interconverting forms.
Not recognizing all possible resonance structures.
Incorrectly assigning formal charges.
Keywords
Resonance
Canonical forms
Resonance hybrid
Delocalization
Conjugation
Π electrons
Lone pair
Formal charge
Stability
Aromaticity
Practice preview
The actual structure of benzene is a resonance hybrid of two Kekulé structures. What is the bond order between carbon atoms in benzene?…
medium
Which of the following species exhibits resonance?…
easy
In the resonance structures of ozone (O3), the bond order between oxygen atoms is:…
medium