Chapter 9, Problem 9.69QE

(a)

Interpretation Introduction

Interpretation:

The possible resonance structures for the following skeleton structure have to be determined. Also, the resonance structure that is the most important has to be determined.

Concept Introduction:

The steps to draw the Lewis structure of the molecule are as follows:

Step 1: Find the central atom and place the other atoms around it. The atom in a compound that has the lowest group number or lowest electronegativity considered as the central atom.

Step 2: Estimate the total number of valence electrons.

Step 3: Connect the other atoms around the central atoms to the central atom with a single bond and lower the value of valence electrons by 2 of every single bond.

Step 4: Allocate the remaining electrons in pairs so that each atom can get 8 electrons.

The formula to calculate formal charge of the atom is as follows:

  $\text{Formal}\text{charge}=\left(\begin{array}{l}\text{number}\text{of}\\ \text{valence}\\ \text{electrons}\end{array}\right)-\left(\left(\begin{array}{l}\text{number}\text{of}\\ \text{lone pairs of}\\ \text{electrons}\end{array}\right)+\left(\frac{1}{2}\right)\left(\begin{array}{l}\text{number}\text{of}\\ \text{shared}\\ \text{electrons}\end{array}\right)\right)$        (1)

Some molecules and ions do not have one unique Lewis structure. The Lewis structures that differ only in the placement of multiple bonds are called resonance structures.

Resonance structures are defined as a set of two or more Lewis structures that collectively describe the electronic bonding. The actual bonding is an average of the bonding in the resonance structures. Also, not all resonance structures contribute equally in every case. Resonance structures that have high formal charges or that place charges of the same sign on adjacent atoms do not contribute to the bonding.

Answer to Problem 9.69QE

The possible resonance structures are,

Structure I is more important.

Explanation of Solution

The given skeleton structure is,

The resonance structures are as follows:

For structure I:

Substitute 5 for valence electrons, 4 for the number of lone pairs of electrons and 4 for the number of shared electrons in equation (1) to calculate the formal charge on first $\text{N}$ atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_1\right)=\left(5\right)-\left(\left(4\right)+\left(\frac{1}{2}\right)\left(4\right)\right)\\ =-1\end{array}$

Substitute 5 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on second nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_2\right)=\left(5\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =+1\end{array}$

Substitute 5 for valence electrons, 4 for number of lone pairs of electrons and 4 for the number of shared electrons in equation (1) to calculate the formal charge on third nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_3\right)=\left(5\right)-\left(\left(4\right)+\left(\frac{1}{2}\right)\left(4\right)\right)\\ =-1\end{array}$

For structure II:

Substitute 5 for valence electrons, 2 for the number of lone pairs of electrons and 6 for the number of shared electrons in equation (1) to calculate the formal charge on first $\text{N}$ atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_1\right)=\left(5\right)-\left(\left(2\right)+\left(\frac{1}{2}\right)\left(6\right)\right)\\ =0\end{array}$

Substitute 5 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on second nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_2\right)=\left(5\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =+1\end{array}$

Substitute 5 for valence electrons, 6 for number of lone pairs of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on third nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_3\right)=\left(5\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-2\end{array}$

For structure III:

Substitute 5 for valence electrons, 6 for the number of lone pair of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on first $\text{N}$ atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_1\right)=\left(5\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-2\end{array}$

Substitute 5 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on second nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_2\right)=\left(5\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =+1\end{array}$

Substitute 5 for valence electrons, 2 for number of lone pairs of electrons and 6for the number of shared electrons in equation (1) to calculate the formal charge on third nitrogen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{N}}_3\right)=\left(5\right)-\left(\left(2\right)+\left(\frac{1}{2}\right)\left(6\right)\right)\\ =0\end{array}$

The possible resonance structures with the formal charges are as follows:

In these resonance structures, structure I have smallest formal charge because Lewis structures that have smallest formal charges are favored.

Hence, structure I is more important.

(b)

Interpretation Introduction

Interpretation:

The possible resonance structures for the following structure have to be determined. Also, the resonance structure that is the most important has to be determined.

Concept introduction:

Refer to part (a)

Answer to Problem 9.69QE

The possible resonance structures are as follows:

All structures are equally important.

Explanation of Solution

The given skeleton structure is,

The resonance structures are as follows:

For structure I:

Substitute 4 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on carbon atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left(\text{C}\right)=\left(4\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =0\end{array}$

Substitute 6 for valence electrons, 4 for number of lone pairs of electrons and 4for the number of shared electrons in equation (1) to calculate the formal charge on first oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_1\right)=\left(6\right)-\left(\left(4\right)+\left(\frac{1}{2}\right)\left(4\right)\right)\\ =0\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on second oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_2\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on third oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_3\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

For structure II:

Substitute 4 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on carbon atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left(\text{C}\right)=\left(4\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =0\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2for the number of shared electrons in equation (1) to calculate the formal charge on first oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_1\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

Substitute 6 for valence electrons, 4 for number of lone pairs of electrons and 4for the number of shared electrons in equation (1) to calculate the formal charge on second oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_2\right)=\left(6\right)-\left(\left(4\right)+\left(\frac{1}{2}\right)\left(4\right)\right)\\ =0\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2for the number of shared electrons in equation (1) to calculate the formal charge on third oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_3\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

For structure III:

Substitute 4 for valence electrons, 0 for number of lone pairs of electrons and 8 for the number of shared electrons in equation (1) to calculate the formal charge on carbon atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left(\text{C}\right)=\left(4\right)-\left(\left(0\right)+\left(\frac{1}{2}\right)\left(8\right)\right)\\ =0\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on first oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_1\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

Substitute 6 for valence electrons, 6 for number of lone pairs of electrons and 2 for the number of shared electrons in equation (1) to calculate the formal charge on second oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_2\right)=\left(6\right)-\left(\left(6\right)+\left(\frac{1}{2}\right)\left(2\right)\right)\\ =-1\end{array}$

Substitute 6 for valence electrons, 4 for number of lone pairs of electrons and 4 for the number of shared electrons in equation (1) to calculate the formal charge on third oxygen atom.

  $\begin{array}{c}\text{Formal}\text{charge}\left({\text{O}}_3\right)=\left(6\right)-\left(\left(4\right)+\left(\frac{1}{2}\right)\left(4\right)\right)\\ =0\end{array}$

Possible resonance structures are as follows:

In these resonance structures, all the structures have the same formal charge. Hence, all the structures are equally important.