Concept of Resonance
According to the concept of resonance whenever a single Lewis structure cannot describe a molecular structure accurately, a number of structures with similar energy, positions of nuclei, bonding and non bonding pairs of electrons are considered to represent the structure. Each such structure is called as canonical structure. A resonance hybrid consists of many canonical structures. All the canonical structures are equally possible to represent the structure of the molecule.
For example, in ozone (O3) molecule, the two canonical structures as shown below and their hybrid represents the structure of O3 more accurately. Resonance is represented by a double headed arrow placed between the canonical structures. There are two canonical forms of O3.
The resonance structures are possible for molecular ions also. For example, consider resonance in CO32- ion:-
The single Lewis structure based on the presence of two single bonds and one double bond between each carbon and oxygen atoms is inadequate to represent the molecule accurately as it represents unequal bonds. According to experimental findings all carbon to oxygen bonds in CO32- are equivalent. Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms as shown in Fig. b.
There are three canonical forms of CO32-.
Structure of CO2 molecule is also an example of resonance, the experimental C-O bond length is found to be shorter than C-O single bond length and longer than C=O bond length and lies intermediate in value between a pure single and a pure double bond lengths. Also the two C=O bond length in the CO2 molecule are equivalent and the properties of the two bonds are also the same. Therefore, a single lewis structure cannot depict the structure of CO2 as a whole and it is best described as a resonance hybrid of the canonical forms given in Fig. c.
In N2O molecule which is a linear molecule, structures with charges on atoms can be written similar to CO2.
Here also the experimental bond length of N-N-bond lies between a double and triple bond and that of N-O bond length lies between a single and a double bond. Therefore N2O exists as a hybrid structure of the two canonical forms with a linear geometry.