Question

How would you calculate the formal charge of ${\mathrm{NH}}_3$?

Answer 1

Formal charge:-

• Formal charge of a compound can be calculated by the formula:

$\mathrm{FC}=\mathrm{V}-\mathrm{N}-\frac{\mathrm{B}}{2}$

• Here, FC= Formal charge, V= number of valence electrons in a free atom, N= total number of non-bonding electrons, B= total number of bonding electrons

Formal charge of ${\mathrm{NH}}_3$:-

• Structure of ${\mathrm{NH}}_3$is:

Step 1: Formal charge of Nitrogen

Here Nitrogen is the free atom and the number of valence electrons of it is 5.

Number of non-bonding electrons is 2 and bonding electrons are 6

$\therefore$Formal charge of Nitrogen is

$$\begin{gathered} \mathrm{FC}=\mathrm{V}-\mathrm{N}-\frac{\mathrm{B}}{2} \\ \Rightarrow \mathrm{FC}=5-2-\left(\frac{6}{2}\right) \\ \Rightarrow \mathrm{FC}=5-5 \\ \Rightarrow \mathrm{FC}=0 \end{gathered}$$

Step 2: Formal charge of Hydrogen

Here hydrogen is the free atom and the number of valence electrons of it is 1.

Number of non-bonding electrons is 0 and bonding electrons are 2

$\therefore$Formal charge of hydrogen is

$$\begin{gathered} \mathrm{FC}=\mathrm{V}-\mathrm{N}-\frac{\mathrm{B}}{2} \\ \Rightarrow \mathrm{FC}=1-0-\left(\frac{2}{2}\right) \\ \Rightarrow \mathrm{FC}=1-1 \\ \Rightarrow \mathrm{FC}=0 \end{gathered}$$

Step 4: Formal charge of ${\mathrm{NH}}_3$

Adding all the three different formal charges of Nitrogen and Hydrogen we get the formal charge of ${\mathrm{NH}}_3$

$\therefore$Formal charge of ${\mathrm{NH}}_3$is

$$\begin{gathered} \mathrm{FC}=0+3\times \left(0\right) \\ \Rightarrow \mathrm{FC}=0 \end{gathered}$$ [$\because$since ${\mathrm{NH}}_3$contains 3 hydrogen atoms]

Therefore, the formal charge of ${\mathrm{NH}}_3$ is $0$