Question

Which of the following statements is correct for $${\text{NO}}_3^{-}$$ ion?
(A) Sum of all formal charges is +1.
(B) Formal charge on one of the oxygen atoms is -2.
(C) Formal charge on nitrogen atoms is +1.
(D) Average formal charge on oxygen atom is $$-{\displaystyle \frac{2}{3}}$$

Answer 1

Hint: $${\text{NO}}_3^{-}$$ ion is nitrate ion. It is obtained when nitric acid ionizes and loses a proton.
Use the following expression to calculate the formal charge on each atom
$$\left(\begin{array}{c}\text{Formal charge}\\ \text{on an atom}\end{array}\right)=\left(\begin{array}{c}\text{Number of}\\ \text{valence electrons}\end{array}\right)-\left(\begin{array}{c}\text{Number of}\\ \text{non bonding electrons}\end{array}\right)-\left[{\displaystyle \frac{\text{1}}{2}}\left(\begin{array}{c}\text{Number of}\\ \text{bonding electrons}\end{array}\right)\right]$$

Complete step by step answer:
Nitrogen atoms have 5 valence electrons. Oxygen atoms have 6 valence electrons. One oxygen atom has additional electrons due to negative charge on nitrate ions. This additional electron is transferred to oxygen when hydrogen-oxygen bond heterolytically breaks in nitric acid to provide an electron.
Draw the Lewis dot structure for nitrate ion.

Nitrogen atoms have 5 valence electrons, 0 nonbonding electrons and 8 bonding electrons. Since nitrogen forms four bonds, it has eight electrons. Calculate the formal charge on nitrogen atom:
$$\begin{array}{l}\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}=5-0-\left[{\displaystyle \frac{1}{2}}\left(8\right)\right]=5-4=+1\\ \mathrm{o}\mathrm{n}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\end{array}$$
Singly bonded atom has 6 valence electrons, 6 nonbonding electrons and 2 bonding electrons. Since oxygen forms one bond, it has two electrons. Calculate the formal charge on singly bonded atom:
$$\begin{array}{l}\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}=6-6-\left[{\displaystyle \frac{1}{2}}\left(2\right)\right]=0-1=-1\\ \mathrm{o}\mathrm{n}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\end{array}$$
Doubly bonded atom has 6 valence electrons, 4 nonbonding electrons and 4 bonding electrons. Since oxygen forms two bonds, it has four electrons. Calculate the formal charge on doubly bonded atom:
$$\begin{array}{l}\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}=6-4-\left[{\displaystyle \frac{1}{2}}\left(4\right)\right]=2-2=0\\ \mathrm{o}\mathrm{n}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\end{array}$$
Let us consider all the options:
(A) Sum of all formal charges is -1 not +1. It is equal to the charge on the nitrate ion. So this option is not correct.
(B) Formal charge on one of the oxygen atoms is 0. For the remaining 2 oxygen atoms, the formal charge is -1 on each oxygen atom. So this option is also not correct.
(C) Formal charge on nitrogen atoms is +1. So this option is correct.
(D) Average formal charge on an oxygen atom is $$-{\displaystyle \frac{2}{3}}$$. This is also correct.
There are three oxygen atoms with a total of formal charges on three oxygen atoms being $$0-1-1=-2$$ .

Hence, the correct options are the options (C) and (D).

Note: The formal charge on an atom in a compound represents the number of electrons gained, lost or shared by that atom to form a bond. When an oxygen atom forms two covalent bonds by sharing of electrons, it has formal charge of 0. When a nitrogen atom forms three covalent bonds and one coordinate bond, it has formal charge of +1. When an oxygen atom accepts an electron, it has formal charge of -1. When an oxygen atom accepts bond pairs of electrons due to heterolytic bond cleavage, it has a formal charge of -1.