theory of resonance
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What is resonance in chemistry?
In chemistry, resonance describes how the actual state of a molecule is a combination of multiple alternative structures, rather than just one. The molecule is said to resonate between these structures or to have a structure that is a resonance hybrid of these structures.
The energy of a resonance hybrid is lower than any of the alternative structures, which stabilizes the molecule. The difference in energy between one of the alternative structures and the resonance hybrid is called resonance energy.
Resonance is used to formulate structures for molecules for which a single structure with single, double, and triple bonds cannot be assigned. The theory of resonance is based on quantum mechanics, where a wave function can represent a stationary state of a system as a weighted sum of wave functions corresponding to hypothetical structures, which leads to a minimum calculated energy for the system.
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Why do some molecules need resonance structures to describe them?
Molecules need resonance structures when a single Lewis structure fails to accurately represent the true distribution of electrons and bonding within the molecule.
Resonance provides a way to describe the molecule's actual state as a hybrid of several alternative structures. This is particularly important when there is ambiguity in the placement of double bonds or when electrons are delocalized over multiple atoms. The blending of these structures is a quantum mechanical phenomenon that evenly spreads double-bond character across participating atoms.
The true configuration becomes an average of individual structures, which leads to greater stability.
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What are resonance structures or resonance forms?
In chemistry, resonance structures, or resonance forms, describe a molecule's normal state as a combination of multiple alternative distinct structures, rather than a single valence-bond structure. The molecule resonates among these structures, resulting in a resonance hybrid. This hybrid's energy is lower than any individual structure, stabilizing the molecule, and the energy difference is the resonance energy.
A classic example is benzene, where the actual configuration is an average of individual structures, making all carbon-carbon bonds equivalent. This concept also applies to molecules like ozone, where resonance spreads double-bond character evenly over the atoms.
Resonance structures must have similar energies, with atoms arranged approximately the same way and the same number of unpaired electrons. The theory is rooted in quantum mechanics, where a wave function represents a stationary state as a weighted sum of wave functions corresponding to hypothetical structures, minimizing the system's calculated energy.
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How are resonance structures shown when drawing molecule diagrams?
In chemistry, the theory of resonance describes how to represent the actual normal state of a molecule using a combination of multiple alternative structures. A molecule resonates among several valence-bond structures, resulting in a structure that is a resonance hybrid of these structures.
Resonance structures are represented by drawing all possible Lewis structures. These structures are separated by a double-headed arrow. This indicates that the true structure of the molecule is a blend of all possible structures, not just one.
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What is the difference between a resonance hybrid and individual resonance structures?
Resonance structures are different ways to represent a molecule where the bonding is delocalized, and a resonance hybrid is the actual structure that combines all possible resonance structures.
Resonance structures are not real depictions of the molecule but rather theoretical representations. The true structure is intermediate between all resonance forms, weighted by their relative stabilities. The resonance hybrid has lower energy than any single resonance structure; this difference is called resonance energy, which stabilizes the molecule.
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How does resonance affect the stability and properties of molecules like benzene?
Resonance significantly affects the stability and properties of molecules like benzene. The theory of resonance describes how the actual state of a molecule is a hybrid of multiple alternative structures, rather than a single structure.
In benzene, the six electrons are delocalized across the entire ring, meaning they are attracted to all six carbon atoms rather than just one or two. This delocalization increases stability, making benzene less reactive than expected for an unsaturated hydrocarbon. The resonance hybrid is more stable than any single contributing structure.
Benzene's structure is planar, with equal carbon-carbon bond distances of 1.40 angstroms, intermediate between single and double bonds. All bond angles are 120°, forming a regular hexagon. This stability is evident in its lower reactivity compared to other unsaturated hydrocarbons.
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theory of resonance, in chemistry, theory by which the actual normal state of a molecule is represented not by a single valence-bond structure but by a combination of several alternative distinct structures. The molecule is then said to resonate among the several valence-bond structures or to have a structure that is a resonance hybrid of these structures. The energy calculated for a resonance hybrid is lower than the energies of any of the alternative structures; the molecule is then said to be stabilized by resonance. The difference between the energies of any one of the alternative structures and the energy of the resonance hybrid is designated resonance energy.
The classic example of the application of the theory of resonance is the formulation of the structure of benzene. The structure of benzene as a six-membered ring of carbon atoms was introduced by the German chemist F.A. Kekule in 1865. To make the structure compatible with the quadrivalence of carbon, he introduced alternating single and double bonds in the ring, and in 1872, in order to account for the fact that no isomers of benzene (no isomeric orthosubstituted benzenes differing in having single or double bonds between the substituted carbon atoms) had been observed, he introduced the idea of an oscillation between structures of the form:
In the years following 1920, several scientists proposed the idea that the true state of the molecule may be intermediate between those represented by several different valence-bond structures. Further clarification of the structure of benzene was provided by a U.S. chemist, Linus Pauling, in 1931 with the proposal that the normal state of the molecule can be represented as a hybrid of the two Kekule structures and the three structures of the form:
The actual configuration of the molecule is a suitable average of the configurations corresponding to the individual structures. Because of resonance the six carbon–carbon bonds are equivalent, in agreement with conclusions derived from experimental measurements. Furthermore, the energy of the resonance structure, calculated from quantum-mechanical considerations, is successfully predicted to be less than the energy of any one of the alternative structures.
The concept of resonance has similarly been used to formulate structures for polynuclear aromatic hydrocarbons, molecules containing conjugated systems of double bonds (e.g., biphenyl, butadiene), free radicals, and other molecules to which no satisfactory single structure in terms of single bonds, double bonds, and triple bonds can be assigned (e.g., carbon monoxide, oxygen). Some general rules are used in the selection of suitable resonance structures for a molecule. These rules are: the structures must have energies of similar magnitudes; the arrangement of the atoms must be approximately the same in all the structures; and the structures must have the same numbers of unpaired electrons.
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The theory of resonance is based on the fundamental principle of quantum mechanics, which states that the wave function representing a stationary state of a system can be expressed as a weighted sum of the wave functions that correspond to several hypothetical structures for the system and that the proper combination is that sum which leads to a minimum calculated energy for the system.