Resonance Energy & Extra Thermodynamic Stability
This is Higher Level (HL) content.
When a molecule or ion can be represented by two or more valid Lewis formulas that differ only in the placement of π electrons and lone pairs, the actual structure is none of them individually. Instead, it is a resonance hybrid - a mathematical average that spreads electron density across the entire conjugated system.
Key Consequences of Delocalisation
- Fractional bond orders - e.g. CO₃²⁻ has a bond order of 1⅓ (not alternating singles and doubles)
- Equal bond lengths - all C-O bonds in carbonate are identical (127 pm, between single 143 pm and double 122 pm)
- Increased stability - delocalisation lowers the overall energy of the system
Common Syllabus Examples
| Species | # Resonance Structures | Bond Order |
|---|---|---|
| O₃ (ozone) | 2 | 1.5 |
| CO₃²⁻ (carbonate) | 3 | 1.33 |
| NO₃⁻ (nitrate) | 3 | 1.33 |
| C₆H₆ (benzene) | 2 Kekulé forms | 1.5 |
Resonance Hybrids are Static (Not Rapidly Flipping)
Resonance structures are not in equilibrium with each other - the molecule does not "flip" between forms. The hybrid is the only real structure. The double-headed arrow (↔) between resonance forms means "is a combination of," NOT "converts between."
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