CONCEPT OF RESONANCE
DR ATUL KUMAR SRIVASTAVA
ASSISTANT PROFESSOR
DEPTT OF CHEMISTRY
MAGADH UNIVERSITY, BODH GAYA
If more than one Lewis structure of a molecules could be drawn, and experimental
properties are inconsistent with any one structure. In order to address this type of
situation, several contributing structures are drawn by changing the position of lone pair
of electron or pi electrons, these structures are called canonical structure or
contributing structure or resonating structures and the concept is termed resonance.
These considered together as an average, and the molecule is said to be represented
by a resonance hybrid in which several Lewis structures are used collectively to
describe its true structure. The resonating structures are connected by double headed
arrows (- - - >). For example
1. Nitrite ion : It can be represented by following resonating structures
The experimental geometry of the nitrite anion, NO 2–, shown on the right, is best rationalized by describing its
structure as a resonance hybrid consisting of two major and equally important contributing forms.
2.
Carbonate ion : It can be represented by following resonating structures
3.
Contributing structures of the carbonate ion
Carbon dioxide :
CO2, has three resonating structures, out of which one is a major contributing structure .
The CO2 molecule has a total of 16 valence electrons , 4 from carbon and 6 from each oxygen
atom.
Here are the three resonance structures for CO2, all accounting for the 16 valence
electrons
In above three structures the relative position of the carbon and oxygen atoms remain the
same but the arrangement of valence electrons are different.
4. Benzene :
The different resonating structures of benzene are given as
In the above two contributing structures of benzene, all single and double bonds are
interchange.
In the hybrid structure on the right, the dashed hexagon in form of circle replaces three
double bonds, and represents delocalized six electrons in a set of
three molecular orbitals of π symmetry.
Rules for writing Resonance structures :
1. All the atoms take part in resonance must be present in same plane.
2. In resonance only the delocalisation of electrons in π bonds or lone pairs but the atoms
never moved.
3. In different resonating structures the structure in which all atoms have complete valence
shells is more stable.
4. In different resonating structures the structure will have the greater the number of covalent
bonds , will have the greater stability since more atoms will have complete octets.
5. In different resonating structures the structure with the least number of formal charges is
more stable.
6. In different resonating structures the structure with the least separation of formal charges will
be more stable.
7. In different resonating structures the structure in which negative charge on the more
electronegative atom will be more stable.
8. None of the resonating structure will be the real structure, but the real structure will be the
hybrid of all resonating structures called resonance hybrid.
Resonance Energy
The resonance gives the extra stability to the molecule. The resonance energy is defined as the
difference in energy between the actual structure of the entity and the most stable of the
hypothetical structures. Resonance energy is the amount of energy needed to convert the true
delocalized structure into that of the most stable contributing structure.
Difference between Resonance and Mesomeric Effect :
Resonance refers to delocalization of electrons in a given system. If the
electron withdrawing or releasing effect is due to a substituent through delocalization of π electrons,
by which various canonical forms can be drawn, is known as mesomeric effect . It is symbolized
by M or R.
Thus we can say that the kind of electron distribution in unsaturated conjugated compounds
with electron-releasing or withdrawing groups or atoms is called mesomeric effect. So
when resonance involves displacement of pi electrons of a multiple bond then the term
mesomerism is usually applied instead of Resonance.