Resonance (chemistry)

Carbonate-ion-resonance-2D

Nitrite

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Benzene delocalization

Furan resonance with arrows

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Delocalization ozone

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Delocalization allyl cation

Resonance in chemistry is a concept used to describe the way some chemical structures can be represented by two or more valid Lewis structures, also known as resonance structures or contributing structures. These structures are used to depict the delocalization of electrons within a molecule, which cannot be accurately represented by a single Lewis structure. Resonance is a critical concept for understanding the stability, shape, and electronic distribution of molecules, especially in organic chemistry.

Overview

In molecules exhibiting resonance, the actual structure is a hybrid of the individual resonance structures. However, it is important to note that the molecule does not oscillate between these structures; rather, the actual electronic structure is a composite that averages the characteristics of each contributing structure. This phenomenon leads to increased stability due to the delocalization of electrons across different parts of the molecule.

Resonance Structures

Resonance structures are drawn with the same arrangement of atoms but differ in the placement of electrons. In creating these structures, certain rules must be followed:

• The positions of the nuclei must remain the same.
• Only the electrons involved in pi bonds or lone pairs can be moved.
• The overall charge of the system must remain constant.

Resonance Hybrid

The actual molecule is better represented by a resonance hybrid, which is a conceptual blend of all the resonance structures. This hybrid reflects the real electron distribution more accurately than any single resonance structure. The resonance hybrid is depicted by drawing a dashed line or a circle in the area where the electrons are delocalized.

Conditions for Resonance

Not all molecules exhibit resonance. Certain conditions must be met, such as:

• The molecule must contain a pi bond adjacent to a double bond or a lone pair of electrons.
• The atoms involved must lie in the same plane, allowing for the overlap of p-orbitals.
• There must be a possibility of drawing two or more valid Lewis structures that differ only in the position of electrons.

Significance of Resonance

Resonance has profound implications in chemistry:

• It explains the stability of molecules that cannot be depicted accurately by a single Lewis structure.
• It helps in understanding the electronic distribution within a molecule, which influences its reactivity and properties.
• Resonance is key to explaining the behavior of many organic compounds, including aromatic compounds like benzene.

Examples

A classic example of resonance is found in the benzene molecule (C6H6), where the delocalization of electrons across the carbon atoms contributes to its unusual stability and is represented by two equivalent structures. Another example is the nitrate ion (NO3-), which has three resonance structures showing the delocalization of the negative charge.