Overview
This lecture introduces resonance structures in molecules, explains how to construct and evaluate them, and provides guidance for determining their stability and contribution to the real molecular structure.
Resonance Structures: Introduction and Ozone Example
- Some molecules can be represented by more than one valid Lewis (Lou) structure; these are called resonance structures.
- Ozone (O₃) has two valid resonance structures, each with different electron arrangements.
- Both resonance structures for ozone have a positive charge on the central oxygen and a negative charge on a terminal oxygen.
- The real structure of ozone is a hybrid of the two resonance structures, with bond orders between single and double and shared negative charge on terminal oxygens.
Rules for Drawing Resonance Structures
- All resonance structures must have the same atomic positions; atoms do not move.
- Every structure must satisfy the octet rule for each atom where possible.
- The total number of electrons and the net charge must remain the same in all resonance structures.
Stability of Resonance Structures
- Structures with smaller charge separation are more stable.
- A negative charge is more stable when located on the more electronegative atom.
- Greater electron delocalization (movement of electrons between atoms) increases molecular stability.
Examples: Nitrate and Other Ions
- Nitrate ion (NO₃⁻) has three identical resonance structures; the real structure is a hybrid where all N–O bonds are equivalent.
- For NC⁻O (possibly a mistyped molecule, likely NCO⁻), three resonance structures exist but are not identical.
- Structures with greater charge separation (multiple positive/negative charges) are less stable.
- Resonance structures with a negative charge on the more electronegative atom (oxygen over nitrogen) are more stable.
Practice Problems
- Draw three resonance structures for carbonate ion (CO₃²⁻); determine if they are identical and which is most stable if not.
- For acetic acid (CH₃COOH), draw all possible resonance structures and determine their relative stabilities.
Key Terms & Definitions
- Resonance structures — Different Lewis structures for the same molecule where only electrons are moved, not atoms.
- Resonance hybrid — The real structure, a weighted average of all valid resonance structures.
- Formal charge — The hypothetical charge assigned to an atom in a Lewis structure.
- Octet rule — Atoms tend to have eight electrons in their valence shell.
- Electron delocalization — Distribution of electrons across more than two atoms, stabilizing the molecule.
Action Items / Next Steps
- Practice: Write resonance structures for CO₃²⁻ and CH₃COOH; identify which are most stable.
- Review: Ensure understanding of formal charges and their impact on resonance stability.