Resonance structures are a way to represent molecules or ions whose electrons cannot be shown accurately with a single Lewis structure. They are especially important when pi electrons or lone pairs are spread over several atoms. Resonance helps explain why some bonds have equal lengths, why charges are distributed, and why certain molecules are more stable than one drawing suggests.
It is a drawing tool for electron delocalization, not a picture of a molecule rapidly switching shapes.
A resonance contributor differs from another contributor only in the placement of electrons, not in the positions of atoms. Curved arrows show how electron pairs move from lone pairs or bonds to form new pi bonds, lone pairs, or charges. The real structure is the resonance hybrid, which combines the electron distribution of all important contributors.
Major contributors usually have full octets, fewer formal charges, and negative charge on more electronegative atoms.
Key Facts
- Resonance contributors have the same atom positions but different electron placement.
- Curved arrows show electron-pair movement, not atom movement.
- Formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons.
- The resonance hybrid is more stable than any single contributor.
- Equivalent resonance contributors contribute equally to the hybrid.
- Bond order in a resonance hybrid can be fractional, such as 1.5 for equivalent single and double bond sharing.
Vocabulary
- Resonance contributor
- One valid Lewis structure that helps describe the delocalized electron arrangement in a molecule or ion.
- Resonance hybrid
- The real electron distribution of a molecule or ion represented as a blend of its resonance contributors.
- Delocalized electrons
- Electrons that are spread over three or more atoms instead of being confined between two atoms or on one atom.
- Curved arrow
- A notation that shows the movement of an electron pair from an electron-rich region to a new location.
- Formal charge
- The charge assigned to an atom in a Lewis structure by comparing its assigned electrons with its valence electrons.
Common Mistakes to Avoid
- Moving atoms while drawing resonance structures is wrong because resonance only changes electron placement, not the molecular skeleton.
- Using a double-headed equilibrium arrow between contributors is wrong because resonance contributors are not separate species in equilibrium.
- Breaking the octet rule for second-row atoms like carbon, nitrogen, oxygen, or fluorine is wrong because these atoms cannot have more than eight valence electrons.
- Choosing the contributor with the most separated charges as the best structure is wrong because major contributors usually minimize formal charge and place negative charge on more electronegative atoms.
Practice Questions
- 1 For ozone, O3, draw the two equivalent resonance contributors and calculate the average O-O bond order in the resonance hybrid.
- 2 For carbonate, CO3^2-, draw the three equivalent resonance contributors and calculate the average C-O bond order.
- 3 Explain why the resonance hybrid of carbonate has three identical C-O bonds even though each resonance contributor shows one C=O bond and two C-O bonds.