   Chapter 8.7, Problem 8.12CYU

Chapter
Section
Textbook Problem

Draw the resonance structures for SCN‒. What are the formal charges on each atom in each resonance structure? What are the bond polarities? Do they agree with the formal charges?

Interpretation Introduction

Interpretation: The resonance structure, formal charge of each atom and bond polarities present in the SCN- molecule should be determined.  It should be identified that whether the formal charges agree with the bond polarities.

Concept Introduction:

Electronegativity: It is defined as the capacity of the atom to abstract the pair of electrons towards itself results to have high negative charge.

• Polar molecule: The molecule with atoms bonded with different electronegativity. Dipole moment is used to measure the polarity of the molecule.
• Polarity of a molecule is measured in term of dipole moment.
• Dipole moment for a polar molecule is non-zero and for a non-polar molecule dipole moment is zero

Formal charge: It is the electrostatic charge that would reside on an atom in a molecule or polyatomic ion if all bonding electron are shared equally between pairs of atoms.

Formal charge calculation: The formal charge for atom in a molecule or ion is calculated based on the Lewis structure of the molecule or ion by following the given equation below:

= NVE-{LPE+12(BE)where,NVE = No.of valence electronsLPE = Lone pair electronsBE = Bond pair electrons

• 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.
• All the possible resonance structures are imaginary whereas the resonance hybrid is real.
Explanation

Resonance structure of SCN- should be drawn by delocalizing the phi bond present in the given molecule as follows,

Formal charge calculation for each atom present in resonance structures are as follows,

S=6-2-12×6 = +1C=4-0-12×8 = 0N =5- 6-12×2 =-2

S=6-4-12×4 = 0C=4-0-12×8 = 0N =5- 4-12×4 =-1

S=6-6-12×2 = -1C=4-0-12×8 = 0N =5- 2-12×6 = 0

C-N bond is more polar bond in the molecule since Δx for C-N is more than C-S

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