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Concept of Resonance
There are certain compounds, which can be represented by more
than one Lewis structures, but none of the structure is able to represent the
molecule in agreement with its experimentally determined parameters. The
phenomenon of resonance is introduced because of the inability of single Lewis
structures to show electron delocalization over the molecule. So Heisenberg
introduced the phenomenon of resonance to explain the properties of certain

Take the example of ozone molecule to explain resonance.

In this case, each oxygen atom has an octet of electrons Acc. To
structure (1) there is one single bond (O-O) and one double bond.
The bond lengths for (O-O) and (O=O) are 148pm and 121pm
respectively. So according to the above structures, we expect the
two bond lengths in the ozone molecule will be different, but it is
found that both the bond lengths are equal i.e. 128pm. This bond
length is intermediate between single and double bonds. It means

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that above Lewis structures do not account for the observed
experimental facts. It is proposed that the actual structure lies in
between the above two structures and is called resonance hybrid
as shown below.

Resonance hybrid structure of ozone with some electrons delocalized
between the atoms.

Resonance structures have no real existence. These are the only theoretical
based. Also, it does not actual molecule exists as a one form for a certain fraction
of time or in another form. The resonating structures are the only an easiest way
of picturing a molecule to account for its properties.

Thus Resonance can be defined as: when a molecule can’t be represented by a
single structure but its characteristic properties can be described by two or more
than two structures, then the actual structure is said to be a resonance hybrid of
these structures.

Conditions/ Rules for writing the resonating structure:

1) Contributing structure should have same atomic positions. They should
differ only in the electronic arrangement.

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c) Resonating structure of Carbon monoxide

d) Resonating structure of phosphate ion

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Explanation of some stru ctures:

1) Sulfur dioxide

Let’s consider the sulfur dioxide, SO2, molecule. Structural studies explain that
both the bonds in SO2 are of equal length and strength. This electronic
distribution is unexplained by either single Lewis structure, but is clearly
illustrated when both Lewis formulae are taken into consideration.

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(1) (2)

(1) With symbols of the elements
(2) Simplest one

It appears from the Lewis structures that benzene molecule it
contains two types of C-C bonds, double and single. But actually all
of C-C bonds of benzene are same, and can be explained why, in
terms of resonance. It is as if the benzene ring were resonating
between the two structures below.

The resonance hybrid of benzene is shown below.

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Implications of resonance are

(1) the actual electron distribution is different than would be expected based on a
single Lewis structure, and

(2) the energy of the actual molecule is lower than expected from a single Lewis
structure. This energy lowering is due to electron delocalization and is called
resonance stabilization. The degree of energy lowering or molecular stabilization
is related to the number of resonance structures and the relative stabilities of the
resonance structures.


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