Resonance Structures

Resonance structures are sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule. In many cases, a single Lewis structure fails to explain the bonding in a molecule/polyatomic ion due to the presence of partial charges and fractional bonds in it. In such cases, resonance structures are used to describe chemical bonding.

Resonance in chemistry could be a manner of describing the bonding in particular molecules or ions by merging many contributory structures or forms, jointly called canonical structures or resonance structures within the theory of valence bonding into a hybrid resonance (or hybrid structure).

Resonance Structures

Resonance in Chemistry

The different resonance structures of the carbonate ion (CO32-) are illustrated above. The delocalization of electrons is described via fractional bonds (which are denoted by dotted lines) and fractional charges in a resonance hybrid.

Resonance Structures of NO2 Ion

In the nitrite ion, the bond lengths of both nitrogen-oxygen bonds are equal. The Lewis dot structures of NO2 highlight a difference in the bond order of the two N-O bonds. The resonance hybrid of this polyatomic ion, obtained from its different resonance structures, can be used to explain the equal bond lengths, as illustrated below.

Resonance Structures of NO2

Resonance Structures of NO2- Ion

The resonance hybrid of NO2 suggests that each oxygen atom holds a partial charge of magnitude -½. The bond length of the N-O bonds is found to be 125 pm.

Resonance Structures of NO3 Ion

Nitrogen is the central atom in a nitrate ion. It is singly bonded to two oxygen atoms and doubly bonded to one oxygen atom. The oxygen atoms that are singly bonded to the nitrogen hold a charge of -1 (in order to satisfy the octet configuration). The central nitrogen atom has a charge of +1 and the overall charge on the nitrate ion is -1. The three possible resonance structures of NO3 are illustrated below.

Resonance Structures of NO3

Resonance Structures of NO3- Ion

If a resonance hybrid of this polyatomic ion is drawn from the set of Lewis structures provided above, the partial charge on each oxygen atom will be equal to -(⅔). The net charge on the central atom remains +1. This resonance hybrid is illustrated below.

Resonance Hybrid of NO3

Resonance Structures of O3

The ozone (O3) molecule consists of a central oxygen atom which is singly bonded to one oxygen atom and doubly bonded to another. There is no net charge on this molecule, but the Lewis structures of this molecule show a +1 charge on the central oxygen and a -1 charge on the singly bonded oxygen. The two resonance structures of the ozone molecule are illustrated below.

Resonance Structures of O3

Resonance Structures of O3

The resonance hybrid of ozone has a +1 charge associated with the oxygen at the centre and a partial charge of -(½) associated with the other oxygen atoms.

Resonance Structures of Benzene

Benzene is a very important aromatic hydrocarbon in organic chemistry. It has the chemical formula C6H6. The molecules of benzene have a cyclic structure consisting of alternating single and double bonds between adjacent carbon atoms. Each carbon atom is also bonded to one hydrogen atom. The two possible resonance structures of benzene are illustrated below.

Resonance Structures of Benzene

Resonance Structures of Benzene

The benzene molecule is stabilized by resonance, the pi electrons are delocalized around the ring structure. This delocalization causes each carbon-carbon bond to have a bond order of 1.5, implying that they are stronger than regular C-C sigma bonds. In the resonance hybrid of benzene, the delocalization of pi electrons is described with the help of a circle inside the hexagonal ring.

In benzene, Kekule’s first suggested two cyclohexatriene Kekule structures that have been taken together, they constitute the general structure as contributing structures. The hexagon replaces three double bonds in the hybrid structure on the right and represents six electrons in a collection of three molecular orbitals with a nodal plane in the molecule plane.

Thus, the resonance structures of some molecules and polyatomic ions are discussed briefly in this article. Learn more about this concept and other related concepts such as hyperconjugation, resonance effect and electron dot formula.

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A solid which floats in H2O, sinks in D2O. Why?

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