Source: Chapter 1, Prof. Forrest Michael, University of Washington
Difficulty: Introductory to Intermediate | Prerequisites: Formal charge and Lewis structures (Lecture 1 notes), electronegativity trends.
Resonance is how organic chemists deal with the reality that a single Lewis structure often cannot capture a molecule's true electron distribution. Instead of one fixed picture, you draw several valid structures and blend them into a "hybrid" that better represents the actual molecule. This lecture also covers how bond length and bond strength relate to each other, to bond order, and to polarity. You need to be comfortable with formal charge (Lecture 1) before this makes sense, because formal charge is the main tool you use to rank resonance contributors.
Resonance structures are different valid Lewis structures for the same molecule. The real molecule is a weighted average (the resonance hybrid), not any single contributor. To rank which contributor matters most, check for octet violations and misplaced charges. For bonds: higher bond order means shorter and stronger; bonds to hydrogen are generally stronger than bonds between heavier atoms; and polarity increases with electronegativity difference.
Resonance structures
Two or more valid Lewis structures for the same molecule, differing only in the placement of electrons (not atoms). Think of them as different "snapshots" of where the electrons could be. The molecule does not flip between them; they all exist simultaneously as a blend.
Resonance hybrid
The weighted average of all resonance contributors. This is the actual structure of the molecule. In simple terms, the hybrid is the real thing; individual resonance structures are just our imperfect attempts to draw it.
Formal charge (review)
The charge assigned to an atom based on how electrons are distributed in a given Lewis structure: valence electrons minus (lone pair electrons + bonds).
Octet rule
The principle that main-group elements tend to bond so that each atom has eight electrons in its valence shell. Violations are possible but reduce a resonance contributor's stability.
Bond length
The distance between two bonded nuclei. Shorter bonds are generally stronger bonds.
Bond strength (bond dissociation energy)
The energy required to break a bond homolytically (into two radicals). Think of it as how much energy you need to pull two atoms apart.
Bond polarity
The unequal sharing of electrons in a bond due to a difference in electronegativity between the two atoms. Greater polarity generally means a stronger bond for a given bond order.
No "Texas carbons": never give carbon more than four bonds. Carbon's octet is strict in organic chemistry.
Place negative formal charges on the more electronegative atom whenever possible. A structure with -1 on oxygen is more stable than the same charge on carbon.
Violating the octet rule is allowed for some elements but greatly reduces a contributor's importance in the hybrid.
Draw curved arrows showing where electrons move. Arrows go from where electrons are to where they are going (from a lone pair or bond to an adjacent bond or atom).
When multiple resonance structures exist, rank them by these criteria (most important first):
Full octets on all atoms beats structures with incomplete octets
Minimise formal charges (structures with fewer or smaller formal charges are more stable)
Negative charges on more electronegative atoms
Positive charges on less electronegative atoms
Three resonance structures were discussed in lecture:
Structure A: H-C=N-H with standard bonding, all octets satisfied
Structure B: violates the octet rule on one atom
Structure C: places charge on the wrong atom
Stability ranking: A > C > B (A contributes most, C somewhat, B hardly at all)
Four resonance structures (A, B, C, D) were drawn:
Structure A: the standard Lewis structure with C=O and C-O-H, all octets full
Structure D: moves electron density but keeps full octets
Structure C: breaks the charge rule (charge on the wrong atom)
Structure B: breaks both the charge rule and the octet rule
Stability ranking: A > D > C > B
The resonance hybrid shows partial double-bond character in both C-O bonds, with partial charges (δ+ on C, δ- on O)
As bond order increases, bond length decreases and bond strength increases
Single bond: longest, weakest
Double bond: intermediate
Triple bond: shortest, strongest
As polarity increases, bond strength increases and bond length decreases
Bonds to hydrogen are generally stronger than bonds between two heavy atoms of the same period
When comparing bond strengths, pay attention to the hybridisation of the atoms involved (sp bonds are shorter and stronger than sp² bonds, which are shorter and stronger than sp³ bonds)
From the lecture's comparison of four structures, the ranking was: C > A > B > D, determined by considering bond order, hybridisation, and polarity together.
Formal charge (for ranking resonance):
FC = V - (N + B)
Bond order, length, and strength relationships:
↑ bond order → ↓ bond length → ↑ bond strength
↑ polarity → ↑ bond strength → ↓ bond length
↑ number of bonds → ↑ strength → ↓ length
These are inverse or direct proportional relationships that you should be able to apply in either direction on an exam.
Resonance is why the peptide bond in proteins is planar and rigid: the C-N bond has partial double-bond character from resonance, which constrains the protein backbone's shape. Bond strength relationships explain why C-F bonds are among the strongest in organic chemistry (high polarity, short bond), which is why fluorinated compounds are so persistent in the environment ("forever chemicals" like PFAS).
Students often think the molecule "flips" between resonance structures. It does not. The resonance hybrid is the one real structure; individual contributors are a limitation of how we draw molecules.
A common mistake is moving atoms when drawing resonance structures. Only electrons move. If you have to move an atom, you are drawing a constitutional isomer, not a resonance structure.
Students sometimes rank resonance contributors solely by the number of formal charges. This matters, but you must also check whether octets are satisfied and whether charges sit on appropriate atoms (negative on electronegative, positive on electropositive).
Students may confuse bond strength with bond energy in everyday terms. In chemistry, bond strength specifically means the energy needed to break the bond homolytically (bond dissociation energy).
⚠️ Expect to be given 3 or 4 resonance structures and asked to rank them by stability. Use the checklist: octets first, then charge minimisation, then charge placement.
⚠️ "Draw all significant resonance structures" is a classic exam prompt. Remember: no Texas carbons, arrows show electron movement, and you can violate the octet rule but you must note it reduces stability.
⚠️ Bond-strength comparison questions will likely ask you to rank bonds by strength or length given information about bond order, polarity, and hybridisation. Know the direction of each relationship.
⚠️ The resonance hybrid for formic acid (or similar carboxylic acids) is a very common exam topic. Be ready to draw it and explain why the two C-O bonds have different lengths in the acid but equivalent lengths in the carboxylate anion.
True or false: Resonance structures differ in the placement of atoms.
False. Only electron placement changes; atoms stay fixed.
Fill in the blank: A "Texas carbon" means carbon has more than ___ bonds.
Four.
True or false: As bond order increases, bond length increases.
False. Higher bond order means shorter bond length.
Fill in the blank: Negative formal charges should be placed on the more ___ atom.
Electronegative.
True or false: Bonds to hydrogen are generally weaker than bonds between two carbon atoms.
False. Bonds to hydrogen are generally stronger.
Q: Draw the resonance hybrid of formic acid (HCO₂H) and explain what it shows about the C-O bond lengths.
A: The hybrid shows partial double-bond character in both C-O bonds. The C=O bond in the dominant structure is slightly longer than a pure double bond, and the C-O single bond is slightly shorter than a pure single bond. In the carboxylate anion (HCO₂⁻), both C-O bonds become equivalent at a bond order of 1.5.
Q: Given three resonance structures where A satisfies all octets with zero formal charges, B violates the octet rule, and C has charges but octets are satisfied with negative charge on oxygen, rank them by stability.
A: A > C > B. Full octets with no formal charges is the most stable. C is next because it maintains octets and places the negative charge on an electronegative atom. B is least important because octet violations drastically reduce stability.
Q: Which bond is stronger, a C-H bond or a C-C bond? Why?
A: A C-H bond is generally stronger. Bonds to hydrogen tend to be stronger than bonds between two heavier atoms of the same period, in part because the small hydrogen atom allows closer approach and greater orbital overlap.
Q: Rank the following in order of increasing bond strength: C-C single bond, C=C double bond, C≡C triple bond.
A: C-C < C=C < C≡C. Bond strength increases with bond order.
Resonance connects directly to acid-base chemistry (Chapter 2): the stability of a conjugate base often depends on how well it can delocalise charge through resonance. Bond strength relationships feed into reaction energetics: whether a reaction is exothermic or endothermic depends on the relative strengths of bonds broken and formed. Hybridisation (covered in the next set of notes) determines bond angles and geometry, which in turn affect bond strength and resonance overlap.
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