Chapter 6.5, Problem 7PPB

Interpretation Introduction

Interpretation:

The resonance structure of thiocyanate ion $\left({\text{NCS}}^{-}\right)$ and its corresponding formal charge should be found.

Concept Introduction:

• Sometimes the chemical bonding of a molecule cannot be represented using a single Lewis structure.  In these cases, the chemical bonding are described by delocalization of electrons and is known as resonance.
• In some molecules, there is possibility of more than one Lewis structure where all the structures are equally acceptable.  One of the acceptable Lewis structures of these molecules is called resonance structure.  All the possible resonance structures are imaginary whereas the resonance hybrid is real.
• These structures will differ only in the arrangement of the electrons not in the relative position of the atomic nuclei.
• While drawing resonance structure of a molecule some rules should be followed where the position, over whole charge and chemical framework remains intact. Also only π and nonbonding electron has been moved in all the three resonance structures

To draw: The resonance structure of the given molecule

Interpretation Introduction

Interpretation:

The formal charges of three resonance structures of thiocyanate ion $\left({\text{NCS}}^{-}\right)$ should be calculated

Concept Introduction

• A formal charge (FC) is the charge assigned to an atom in a molecule, irrespective of relative electronegativity by thinking that electrons in all chemical bonds are shared equally among atoms.
• This method is used to identify the most probable Lewis structures if more than one possibility exists for a compound.
• The Lewis structure with formal charge on each of the atoms close to zero is taken as the most plausible structure.
• Formal charge of an atom can be determined by the given formula.

$\text{Formal}\text{charge}\left(\text{F}\text{C}\right)\text{=}\left(\begin{array}{l}\text{no}\text{.of}\text{valence}\\ \text{electron}\text{in}\text{atom}\end{array}\right)-\frac{\text{1}}{\text{2}}\left(\begin{array}{l}\text{no}\text{.of}\text{bonding}\\ \text{electrons}\end{array}\right)-\left(\begin{array}{l}\text{no}\text{.of}\text{non-bonding}\\ \text{electrons}\end{array}\right)$

To find: The formal charges of the resonance structures of thiocyanate ion $\left({\text{NCS}}^{-}\right)$