Résonance et structures hybrides

Resonance structures are multiple Lewis structures used to represent molecules where electrons are delocalized across bonds or atoms rather than localized to one position. Some compounds cannot be accurately shown by a single Lewis structure because the actual electron distribution is an average of several possible arrangements. Resonance structures use double-headed arrows to indicate this averaging, not equilibrium between forms.

Resonance structures are identified through electron pushing, a technique using curved arrows to map how valence electrons move between bonds and atoms. Curved arrows show lone pairs transforming into bonds and vice versa, revealing delocalized electron regions. This systematic approach helps visualize all possible electron distributions for a molecule while keeping atomic positions unchanged.

A resonance hybrid is the actual electronic structure of a molecule, representing the weighted average of all its resonance structures. Unlike individual resonance structures, a resonance hybrid never fluctuates between forms; it is always the average distribution shown by all contributing structures. The hybrid explains why experimental measurements show bond properties intermediate between single and double bonds.

Nitrite (NO₂⁻) and carbonate (CO₃²⁻) ions cannot be represented by a single Lewis structure where all atoms have equivalent bonds. Experiments show both N–O bonds in nitrite are identical in length and strength, yet a single structure would show one single and one double bond. Resonance explains this equivalence: the actual structure is the average of multiple forms, making all bonds identical.

Resonance stabilization occurs when electron delocalization lowers a molecule's potential energy below that of any theoretical non-delocalized structure. Molecules with resonance structures are more stable than predicted by single Lewis structures. This extra stability results from the spreading of electron density across multiple atoms or bonds, reducing electron-electron repulsion.

Contributing structures lower in energy more closely resemble the actual molecular structure. Structures are more stable when all atoms have filled valence shells, contain more covalent bonds, minimize formal charges, and place negative charges on more electronegative atoms and positive charges on less electronegative atoms. These preferences help predict which resonance forms dominate the hybrid structure.

Double bonds are shorter and stronger than single bonds between the same atoms because they involve more electron density between nuclei. However, in resonance hybrids, bonds intermediate between single and double show intermediate lengths and strengths. This explains experimental observations like identical bond properties in nitrite, where the hybrid averages single and double bond character.