Resonance, Resonance Energy, Bond Order formula, Important Topics For JEE Main 2024

Resonance : The concept of resonance was introduced by Heisenberg and later developed by Pauling and Ingold to explain the properties of certain molecules. It has been found that the observed properties of certain compounds cannot be satisfactorily explained by writing a single Lewis structure. The molecule is then supposed to have many structures, each of which can explain most of the properties of the molecule, but no one can explain all the properties of the molecules.

The actual structure is in between of all these contributing structures and is called resonance hybrid, and the different individual structures are called resonating structures or canonical forms. This phenomenon is called resonance.

Resonance Example

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Resonating structures Resonance Hybrid

The resonance hybrid of the two Lewis structures makes both the S – O bond lengths equal in size. Let us discuss resonance in ozone. According to its resonance structure it should have one single bond (O–O = 1.48 Å) but experiments show that both the bonds are same which can be proved by its resonance hybrid as shown below.

• Resonance stabilises the molecule as the energy of the resonance hybrid is less than the energy of any single canonical structure.

• Resonance averages the bond characteristics as a whole.

• The canonical forms have no real existence (These are imagery structures).

Resonance Bond Order formula

Resonance Bond Order : To calculate bond order in the polyatomic molecule or ion, we use the following formula:

Bond order =

Resonance Bond Order Example

Resonating Structures

(i) Azide Ion (N ^3– ):

The structures II and III contribute equally and the molecule has almost double bond character in each N–N bond.

(ii)

(iii)

(iv)

Resonance Energy

• Resonance Energy = Actual bond energy - Energy of most stable resonating structure.

• Stability of molecule ∝ Resonance energy.

• More is the no. of covalent bonds in a molecule, more will be its resonance energy.

• Resonance energy ∝ number of resonating structures