Chapter 6, Problem 41QRT

(a)

Interpretation Introduction

Interpretation:

TheLewis structureof Acetaldehyde and a formal charge of each atom have to be written.

Concept Introduction:

A Lewis structure is also known as Lewis dot diagrams or electron dot structures. The bond between atoms and lone pairs of electrons that is present in the molecule.  Lewis structure represents each atom and their position in structure using the chemical symbol.  Excess electrons forms the lone pair are given by pair of dots, and are located next to the atom.

The formula for the formal charge can be written as

  $\text{Formal}\text{charge=}\text{Valence}\text{electrons-lone}\text{pair}\text{electrons}\text{-}\frac{\text{Bonding}\text{electrons}}{\text{2}}$

Explanation of Solution

The correct Lewis structure of the Acetaldehyde can be drawn as,

Draw a chart:

  $\overline{\begin{array}{l}C-C=HO\\ \overline{\underset{\_}{\begin{array}{l}\text{Valence}\text{electrons}4416\\ \text{Lone}\text{pair}\text{electrons}0004\\ \text{Bonding}\text{electrons}8824\\ \text{Formal}\text{charge}4-(0+4)=04-(0+4)=01-(0+1)=\text{0}6-(4+2)=0\end{array}}}\end{array}}$

The total number of valence electron is $18.$  oxygen atom wants to one multiple bonds.

(b)

Interpretation Introduction

Interpretation:

The Lewis structure for ${\text{N}}_{\text{3}}^{\text{-}}\text{ion}$ and a formal charge of each atom has to be written.

Concept Introduction:

Refer Part (a).

Explanation of Solution

There are three correct possible Lewis structure for ${\text{N}}_{\text{3}}^{\text{-}}\text{ion}$ can be drawn as,

First structure:

Draw a chart:

  $\overline{\begin{array}{l}N==N=\\ \overline{\underset{\_}{\begin{array}{l}\text{Valence}\text{electrons}55\\ \text{Lone}\text{pair}\text{electrons}40\\ \text{Bonding}\text{electrons}48\\ \text{Formal}\text{charge}5-(4+2)=-15-(0+4)=+1\end{array}}}\end{array}}$

The total number of valence electron is $16.$

Second structure:

    

Draw a chart:

$\overline{\begin{array}{l}\text{Left-most}\text{N}\text{Middle}\text{N}\text{Right-most}\text{N}\\ \overline{\underset{\_}{\begin{array}{l}\text{Valence}\text{electrons}555\\ \text{Lone}\text{pair}\text{electrons}206\\ \text{Bonding}\text{electrons}682\\ \text{Formal}\text{charge}5-(2+3)=05-(0+4)=+15-(6+1)=-2\end{array}}}\end{array}}$

Third structure:

Draw a chart:

    $\overline{\begin{array}{l}\text{Left-most}\text{N}\text{Middle}\text{N}\text{Right-most}\text{N}\\ \overline{\underset{\_}{\begin{array}{l}\text{Valence}\text{electrons}555\\ \text{Lone}\text{pair}\text{electrons}602\\ \text{Bonding}\text{electrons}286\\ \text{Formal}\text{charge}5-(6+1)=-25-(0+4)=+15-(2+3)=0\end{array}}}\end{array}}$

Therefore, the first of these three structures is the best one because the formal charge on atoms in that structure is lesser value $\text{(-1,}+1)$ than compared to other two structures $\text{(-2,}+1)$

(c)

Interpretation Introduction

Interpretation:

The Lewis structure of ${\text{CH}}_{\text{3}}\text{CN}$ and a formal charge of each atom has to be written.

Concept Introduction:

Refer Part (a).

Explanation of Solution

The correct Lewis structure of the ${\text{CH}}_{\text{3}}\text{CN}$ can be drawn as,

Draw a chart:

  $\overline{\begin{array}{l}\text{C}\text{H}\text{N}\\ \overline{\underset{\_}{\begin{array}{l}\text{Valence}\text{electrons}415\\ \text{Lone}\text{pair}\text{electrons}002\\ \text{Bonding}\text{electrons}826\\ \text{Formal}\text{charge}4-(0+4)=01-(0+1)=05-(2+3)=\text{0}\end{array}}}\end{array}}$

The total number of valence electron is $16.$ Carbonand Nitrogen atom has triple bonds.

Hence, the formal charge of the ${\text{CH}}_{\text{3}}\text{CN}$ is zero.