Question

Draw the Lewis structure with lowest formal charges, and determine the charge of each atom in

(a) OCS

(b) NO.

Short answer

The Lewis structure of

a. OCS = Linear shape with central atom C-atom having -1 charge and have +2 formal charges.

b. NO = Linear shape with central atom N-atom having +3 charge and have +1 formal charge.

Step-by-step solution

Lewis Structure

A Lewis structure can be defined as the basic structure of a molecule which used valence electron of each atom involved.In Lewis’s structure, the least electronegative atom will be the central atom.

Formal Charge is a theoretical concept which is also called fake charge. It is the charge which is dispersed on the whole molecule, not on the single atom.

Formal Charge = Valence electron – Lone pair electron – ½ (Bonded electron)

Explanation

C-atom belongs to group 14 having 4 electrons in the valence and requires to give or take 4 electrons to fulfil its octet whereas O-atom and S-atom belong to group 16 having 6 electrons in the valence shell and require 2 electrons to fulfil the octet. In Lewis’s structure, the octet of C-atom is completed by the two electrons provided by O-atom and two electrons from S-atom.

Formal Charge of each atom:

$\begin{array}{l}\mathbf{F}\mathbf{}\mathbf{or}\mathbf{}\mathbf{O}\mathbf{-}\mathbf{atom}\mathbf{,}\mathbf{}\mathbf{formal}\mathbf{}\mathbf{charge}\mathbf{=}\mathbf{6}\mathbf{–}\mathbf{6}\mathbf{–}\frac{1}{2}\left(2\right)\mathbf{=}\mathbf{-}\mathbf{1}\\ \mathbf{For}\mathbf{}\mathbf{C}\mathbf{-}\mathbf{atom}\mathbf{,}\mathbf{}\mathbf{formal}\mathbf{}\mathbf{charge}\mathbf{=}\mathbf{4}\mathbf{–}\mathbf{0}\mathbf{–}\frac{1}{2}\left(2\right)\mathbf{=}\mathbf{+}\mathbf{2}\\ \mathbf{For}\mathbf{}\mathbf{S}\mathbf{-}\mathbf{atom}\mathbf{,}\mathbf{}\mathbf{formal}\mathbf{}\mathbf{charge}\mathbf{=}\mathbf{6}\mathbf{–}\mathbf{6}\mathbf{–}\frac{1}{2}\left(2\right)\mathbf{=}\mathbf{-}\mathbf{1}\end{array}$

Simplification

N belongs to group 15 having 5 electrons in the valence required to give or take 3 electrons to fulfil its octet, O-atom belongs to group 16 having 6 electrons in the valence shell and requires 2 electrons to fulfil the octet. In Lewis’s structure, the N-atom needs 3 electrons but oxygen gives 2 electrons to form a double bond.

Formal Charge of each atom:

$\begin{array}{l}\mathbf{For}\mathbf{}\mathbf{O}\mathbf{-}\mathbf{atom}\mathbf{,}\mathbf{}\mathbf{formal}\mathbf{}\mathbf{charge}\mathbf{=}\mathbf{6}\mathbf{–}\mathbf{6}\mathbf{–}\frac{1}{2}\left(2\right)\mathbf{=}\mathbf{-}\mathbf{1}\\ \mathbf{For}\mathbf{}\mathbf{N}\mathbf{-}\mathbf{atom}\mathbf{,}\mathbf{}\mathbf{formal}\mathbf{}\mathbf{charge}\mathbf{=}\mathbf{5}\mathbf{–}\mathbf{3}\mathbf{–}\frac{1}{2}\left(2\right)\mathbf{=}\mathbf{+}\mathbf{1}\\ \end{array}$