Resonance Structures - Practice Questions & MCQ

Bond Order

Bond order is defined as the number of bonds between the two atoms in a molecule. For example, in H-H or H₂ molecule, there is only a single bond present thus its bond order is 1. Further, in O=O or O₂, the bond order is 2 as it has 2 bonds between the oxygen atoms.

Resonance

The nitrite anion can have two possible structures with the atoms in the same positions. The electrons involved in the N–O double bond, however, are in different positions as shown below.

If nitrite ions do indeed contain a single and a double bond, we would expect for the two bond lengths to be different. A double bond between two atoms is shorter than a single bond between the same two atoms. Experiments show, however, that both N–O bonds in NO²⁻ have the same strength and length, and are identical in all other properties.

It is not possible to write a single Lewis structure for NO²⁻ in which nitrogen has an octet and both bonds are equivalent. Instead, we use the concept of resonance: if two or more Lewis structures with the same arrangement of atoms can be written for a molecule or ion, the actual distribution of electrons is an average of that shown by the various Lewis structures. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO²⁻ is the average of a double bond and a single bond. We call the individual Lewis structures resonance forms. The actual electronic structure of the molecule is called a resonance hybrid of the individual resonance forms. A double-headed arrow between Lewis structures indicates that they are resonance forms. Thus, the electronic structure of the NO²⁻ ion is shown as:

Resonance Hybrid
It is the average of the resonance forms shown by the individual Lewis structures or canonical structures.