Q10.2P - Chemistry: Molecular Nature Of Matter And Change

Question

When is a resonance hybrid needed to adequately depict the bonding in a molecule? Using  ${\mathbf{NO}}_{\mathbf{2}}$ as an example, explain how a resonance hybrid is consistent with the actual bond length, bond strength, and bond order.

 

Step-by-Step Solution

Answer

Resonance hybrid is used to describe the delocalization of electrons in the unsaturated molecule. The resonance hybrid is needed to adequately depict the bonding in a molecule because it shows the resultant bonding between two atoms from all the resonating structures.

 

In ${{\mathbf{NO}}_2}^{-}$  Nitrite ion, the bond length of both N-O bonds is equal because of resonance.

In ${{\mathbf{NO}}_2}^{-}$  Nitrite ion, the bond strength increases for the single bond and decreases for the double bond.

In  ${{\mathbf{NO}}_2}^{-}$ Nitrite ion, the bond order is fractional in resonance hybrid.

 

Resonance Structure.

Resonance can be defined as the stability in the resonance hybrid that comes from the different resonating structures of the molecule.

Conjugated unsaturation is required for the resonance to take place.

 

The Resonance structure is used to describe the delocalization of electrons. The resonance is the hybrid structure of all the contributing structures showing different types of bonding. The hybrid structure consists of all the features of bonding in the molecule. 

Explanation

In ${\mathbf{NO}}_2$  molecule, there are 4 electrons on one oxygen atom and 6 electrons available on another oxygen atom available, and two electrons available on the nitrogen atom as shown in the below resonating structures. 

 

In the Lewis structure of ${{\mathbf{NO}}_2}^{-}$  Nitrite ion, the bond length of both N-O bonds is equal because of the continuous changing of single and double bonds.

 

Resonant hybrids of this polyatomic ion, obtained from different resonance structures, can be used to account for the same bond length. The bond length in the resonance consists of the feature of saturated bond (single- bond) and unsaturated bond (double bond) that keeps changing.

 

Here, in resonance, the bond length of a single bond is having some properties of a double bond, i.e., somewhat smaller, whereas the double bond consists of the properties of a single bond, i.e., somewhat longer than the double bond.

The resonance hybrid of ${{\mathbf{NO}}_2}^{-}$  suggests that each O-atom holds a partial negative charge i.e., of the magnitude  $-\frac{1}{2}$ and, the bond length of the N-O bond is 125 pm.

 

 

 

Bond strength increases with the increasing number of bonds between two atoms.

Here, in resonance, the bond strength increases for the single bond as it has some properties of a double bond but the bond strength decreases for the double bond as the force of attraction between the atoms decreases as it has some properties of a single bond.

 

The bond order is the number of bonds present in the structure between two atoms.

Here, in resonance, the bond order of the molecule becomes fractional as the single bond and double bonds get some properties of each other.