Question

Draw a Lewis structure for a resonance form of each ion with the lowest possible formal charges, show the charges, and give oxidation numbers of the atoms:

(a)${{\mathbf{AsO}}_4}^{3-}$

(b) ${{\mathbf{ClO}}_2}^{-}$

Short answer

The Lewis structure of

a.${{\mathbf{AsO}}_4}^{3-}$= Tetrahedral shape having formal charge of the central atom 0 and the oxidation number of As is +7.

b.${{\mathbf{ClO}}_2}^{-}$= Bent shape having formal charge of the central atom 1 and the oxidation number of Cl is +4.

Step-by-step solution

Lewis Structure

A Lewis structure can be defined as the basic structure of a molecule that used the 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 that 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)

The Oxidation number is generally representing the gain and loss of the valence electron of an atom in a molecule.

Explanation

Br-atom belongs to group 17 having 7 electrons in the valence required to give or take 1 electron to fulfill its octet whereas O-atom belongs to group 16 having 6 electrons in the valence shell and requires 2 electrons to fulfill the octet. In Lewis’s structure, the octet of Br-atom is completed by the 5 electrons provided by O-atom and one lone pair electron from the molecule.

Formal Charge of each atom:

$\mathbf{Formal}\mathbf{}\mathbf{charge}\mathbf{}\mathbf{on}\mathbf{}\mathbf{As}\mathbf{-}\mathbf{atom}\mathbf{=}\left(\mathbf{5}\mathbf{–}\mathbf{0}\mathbf{–}\frac{10}{2}\right)\mathbf{=}\mathbf{0}$

Oxidation number of the ${{\mathbf{AsO}}_4}^{3-}$is +7.

Simplification

Cl-atom belongs to group 17 having 7 electrons in the valence shell and requires 1 electron to fulfill the octet and O-atom belongs to group 16 having 6 electrons in the valence shell and requires 2 electrons to fulfill the octet. In Lewis structure, Octet of Cl-atom is fulfilled by the 2 electrons from 1 O-atom and 1 electron from 1 O-atom forming double and single bond respectively.

Formal Charge of each atom:

$\mathbf{Formal}\mathbf{}\mathbf{charge}\mathbf{}\mathbf{on}\mathbf{}\mathbf{Chlorine}\mathbf{=}\mathbf{7}\mathbf{–}\mathbf{4}\mathbf{–}\frac{4}{2}\mathbf{=}\mathbf{1}$

The oxidation number of the${{\mathbf{ClO}}_2}^{-}$is +4.