Question

Assign formal charges to the atoms in each of the following molecules:

a.

b.

c.

Short answer

a.

The formal charge on all H in ${\mathrm{CH}}_3$is 0

The formal charge on C in ${\mathrm{CH}}_3$is 0

The formal charge on O is -1

The formal charge on N is +1

b.

The formal charge on all H in ${\mathrm{CH}}_3$ is 0

The formal charge on C in ${\mathrm{CH}}_3$ is 0

The formal charge on N at left is -1

The formal charge on N in the middle is +1

The formal charge on N at the rightmost is 0.

c.

The formal charge on all H in ${\mathrm{CH}}_3$ is 0.

The formal charge on C in ${\mathrm{CH}}_3$ is 0.

The formal charge on N at left is 0.

The formal charge on N in the middle is +1.

The formal charge on N at the rightmost is -1.

Step-by-step solution

Formal charge

The formal charge is the electric charge present on an atom in a molecule. It indicates magnitude and charge, positive or negative.

Calculation of formal charge

The formal charge on an atom is equal to the number of valence electrons in a neutral and isolated atom minus the number of electrons present in its bonded atom form in a molecule. The number of electrons in a bonded atom is calculated as half the number of bonding electrons plus the nonbonding, that is, lone-pair electrons.

$\begin{array}{c}\text{Formalcharge=}\left(\begin{array}{l}\text{Numberof}\\ \text{valenceelectrons}\\ \text{infreeatom}\end{array}\right)-\left(\begin{array}{l}\text{Numberof}\\ \text{valenceelectrons}\\ \text{inbondedatom}\end{array}\right)\\ =\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)\end{array}$

. Calculation of formal charge on all H in CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=1-\left(\frac{2}{2}+0\right)\\ =1-1\\ =0\end{array}$

(a) Calculation of formal charge on N

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Calculation of formal charge on O

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=6-\left(\frac{2}{2}+6\right)\\ =6-7\\ =-1\end{array}$

Calculation of formal charge on all C in CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

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

 (b) Calculation of formal charge on N bonded with  CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{4}{2}+4\right)\\ =5-6\\ =-1\end{array}$

Calculation of formal charge on N in the middle

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Calculation of formal charge on all C in CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

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

Calculation of formal charge on rightmost N

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{6}{2}+2\right)\\ =5-5\\ =0\end{array}$

 (C):   Calculation of formal charge on N bonded with  CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{2}{2}+4\right)\\ =5-5\\ =0\end{array}$

Calculation of formal charge on N in the middle

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Calculation of formal charge on all C in CH3

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

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

Calculation of formal charge on the rightmost N

Using the above formula:

$\text{Formalcharge}=\left(\begin{array}{l}\text{Numberof}\\ \text{valence}\\ \text{electron}\\ \text{infreeatom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{bondingelectron}}{2}\text{+Numberofnonbondingelectron}\right)$

$\begin{array}{c}=5-\left(\frac{4}{2}+4\right)\\ =5-6\\ =-1\end{array}$