Question

Two major resonance structures are possible for the following anion. One resonance form is given below, but it is incomplete. Complete the given structure by adding nonbonding electrons and formal charges. Draw the remaining structure, including nonbonding electrons and formal charges. Omit curved arrows. Which structure contributes most to the hybrid? Both contribute equally. The structure with the negative charge on nitrogen. The structure with the negative charge on carbon.

Answer 1

Concepts and reason

The incomplete resonance structure of an anion is given. The structure needs to be completed and its other resonating structure needs to be given.

Complete the structure by adding lone pairs to the electron to complete their octet and then adding formal charge by using its formula.

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:

Features of resonance:

• Each contributing structure showing resonance has the same bonding and non-bonding electrons.

• Resonance involves the movement of electrons and not the atoms.

• A double headed arrow is used to represent different contributing structures.

• 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.

Formal charge: It is the charge possessed by an atom in a molecule.

The calculation of formal charge is as follows:

${\rm{Formal}}\;{\rm{charge}}\;{\rm{ = }}\;{{\rm{n}}_{\rm{v}}}\; - \;\frac{{{{\rm{n}}_{\rm{b}}}}}{2} - {{\rm{n}}_{\rm{l}}}$

Where ${{\rm{n}}_{\rm{v}}}$ are the number of valence electrons present in neutral atom.

${{\rm{n}}_{\rm{b}}}$ are the number of bonded electrons.

${{\rm{n}}_{\rm{l}}}$ are the number of non-bonding electrons or lone pair electrons.

Complete the octet of each individual atom by including lone pairs (non-bonding electrons) as follows:

The formal charge on nitrogen is -1 whereas on all the other atoms that is carbon and hydrogen, the formal charge is 0. This is the complete resonating structure of the given anion as shown below:

The other resonating structure of the given anion is as follows:

The formal charges in structure II are as follows:

N, ${{\rm{C}}_1}{\rm{,}}\;{{\rm{C}}_3}$ , and H have no charge whereas ${{\rm{C}}_2}$ has a charge of -1.

The structure with negative charge on nitrogen contributes more to the hybrid structure.

Ans:

The resonance structures of the given anion are as follows:

Answer 2