Question

1. Identify the major and minor resonance contributor(s) for the azide anion, N3–.

major contributor(s):

A.
B.
C.
D.
E.

minor contributor(s):

A.
B.
C.
D.
E.

2. Identify the major and minor resonance contributor(s) for the formate anion, HCO2–.

major contributor(s):

A.
B.
C.
D.

minor contributor(s):

A.
B.
C.
D.

Answer 1

Concepts and reason

Different contributing structures of azide ion and formate anion are given. The major and minor contributing structures among the given compounds needs to be given. Consider the octet rule and charge separation for deciding the major and minor contributing structures.

Fundamentals

Resonance: The representation of some molecules can be done through several Lewis structures. These different structures of the same molecule are known as contributing structures. A hybrid of the contributing structures represents the real structure of the molecule and is known as resonance hybrid.

Such molecules having different Lewis structures exhibit resonance. Resonance occurs through the delocalization of lone pairs over various atoms within the molecule. Consider the example of nitrite ion:

Stability of the contributing structures:

• The contributing structures having a complete octet or maximum number of covalent bonds are the most stable and contributes maximum towards the resonance hybrid.

• A neutral contributing structure is given preference over structures having formal charge.

• The structures with lesser formal charges and lesser charge separation are more stable.

• The contributing structures having negative charge on more electronegative atom is more stable.

• The contributing structures that have opposite formal charges on adjacent atoms are more stable.

Part 1.

The structures D and E of azide anion are incorrect among the given structures. Hence, the structures D and E are eliminated.

The major structure is C and the minor structures are A and B.

Part 2

The structure D of formate anion is incorrect among the given structures. Hence, the structure D is eliminated.

The major contributing structures are A and B and the minor contributing structure is C.

Ans: Part 1

The major and minor resonance contributors for the azide anion are as follows: