Overview
This lecture introduces the concept of resonance in chemistry, explains how resonance structures are drawn, and discusses their significance in understanding molecular stability.
Introduction to Resonance
- Resonance describes the delocalization of electrons in molecules with multiple valid Lewis structures.
- Resonance structures are different Lewis structures for a molecule that differ only in the position of electrons, not atoms.
- The actual structure of the molecule is a hybrid of all possible resonance structures.
Drawing Resonance Structures
- To draw resonance structures, move only the electrons (lone pairs and π bonds), not the position of atoms.
- Use double-headed arrows (↔) to indicate resonance between structures.
- All resonance structures must obey the rules of valency and maintain the overall charge.
- Only electrons adjacent to π bonds or lone pairs on atoms next to double bonds can be moved in resonance.
Significance of Resonance
- Resonance stabilizes molecules by spreading electron density over multiple atoms.
- The resonance hybrid represents the true structure and is more stable than any individual resonance form.
- Resonance explains properties such as bond lengths and molecular reactivity.
Rules and Guidelines
- Resonance structures should have valid Lewis structures and the same arrangement of atoms.
- The most significant resonance structures have the least formal charge, full octets, and negative charges on electronegative atoms.
- Minor contributors may violate these preferences but help describe electron delocalization.
Examples
- Classic examples include the carbonate ion (CO₃²⁻), benzene (C₆H₆), and the nitrate ion (NO₃⁻).
- In each example, electrons are delocalized over multiple atoms, as represented by multiple resonance forms.
Key Terms & Definitions
- Resonance — The delocalization of electrons in a molecule represented by two or more valid Lewis structures.
- Resonance Structure — One of multiple Lewis structures differing only by the placement of electrons.
- Resonance Hybrid — The actual structure, averaging all resonance forms, that reflects real electron distribution.
Action Items / Next Steps
- Practice drawing resonance structures for nitrate, benzene, and carbonate ions.
- Review the rules for identifying valid resonance contributors.