Question

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

Short answer

Answer

The formal charge on C is 0.

The formal charge on N bonded with C is +1

The formal charge on N bonded with N is -1

Step-by-step solution

Formal charge

Formal charge means electric charge present on an atom in a molecule. It indicates the magnitude and whether the charge is 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.

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

 Step 3: Calculation of formal charge on C

Structure showing bond electrons

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=4-}\left(\frac{8}{2}\text{+2}\right) \\ \text{=4-4} \\ =0 \end{gathered}$$

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:

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