Question

Question: Calculate the formal charges on the atoms shown in red.

Short answer

Answer

The formal charge on C is -1

The formal charge on N bonded with C is +1

The formal charge on N bonded with N is 0

Step-by-step solution

Formal charge

Formal charge is the electric charge present on an atom in a molecule. It shows the magnitude and if a charge is positive or negative.

Calculation of formal charge

Formal charge on an atom is calculated as 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 lone-pair electrons.

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{bondedatom}\end{array}\right) \\ \text{=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \end{gathered}$$

Calculation of formal charge on C

Structure showing bond electrons

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=4-}\left(\frac{6}{2}\text{+2}\right) \\ \text{=4-5} \\ =-1 \end{gathered}$$

Calculation of formal charge on N bonded with C:

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=5-}\left(\frac{8}{2}\text{+2}\right) \\ \text{=5-4} \\ =1 \end{gathered}$$

Calculation of formal charge on N bonded with N:

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=5-}\left(\frac{6}{2}\text{+2}\right) \\ \text{=5-5} \\ =0 \end{gathered}$$