   Chapter 8, Problem 20PS

Chapter
Section
Textbook Problem

The following molecules or ions all have three oxygen atoms attached to a central atom. Draw a Lewis structure for each one, and then describe the electron-pair geometry and molecular geometry around the central atom. Comment on similarities and differences in the series. (a) CO32− (b) NO3− (c) SO32− (d) ClO3−

Interpretation Introduction

Interpretation: The Lewis structure, electron pair and molecular geometry for the given set of molecules should be determined also that their similarities and differences should be compared.

Concept Introduction:

• Lewis structures are diagrams that represent the chemical bonding of covalently bonded molecules and coordination compounds.
• It is also known as Lewis dot structures which represent the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
• The Lewis structure is based on the concept of the octet rule so that the electrons shared in each atom should have 8 electrons in its outer shell.

Lewis structure for any molecule is drawn by using the following steps,

First the skeletal structure for the given molecule is drawn then the total number of valence electrons for all atoms present in the molecule is determined

The next step is to subtract the electrons present in the total number of bonds present in the skeletal structure of the molecule with the total valence electrons such that considering each bond contains two electrons with it.

Finally, the electrons which got after subtractions have to be equally distributed such that each atom contains eight electrons in its valence shell.

VSEPR Theory:

As the name itself indicates that the basis for this theory is the electron pair that is bonded electron present in either single or double bonds or lone pair electrons, present in the valence shell tends to repel each other which then tend to be in position in order to minimize the repulsions. The steps involved in the theory in describing the geometry is as follows,

• The first step is to draw the correct Lewis structure for the molecule.
• Then, the electron domain around the central atom should be counted and the geometry that matches with that type of domain in VSEPR should be determined.
• Finally, the geometry is predicted by using the orientation of atoms.

The molecules with considering the domains of type AB2 will tend to have shape like linear or bent if the central atom have lone pair of electrons with it, type AB3 will have shape like trigonal planar, type AB4 will have shape like tetrahedral or square planar, type AB5 will have trigonal bipyramidal and AB6 will have shape like octahedral respectively.

Electron Domain: In VSEPR theory, both the lone pair and the bonded pair are together considered as electron domain regardless of the type of bond in which the bonded pair presents.

Electron-Pair geometry: It is the geometry obtained by considering all valence electrons and bond pairs around central atom.

Molecular geometry: It is the geometry obtained by considering only the directly bonded atoms with the central atom.

Explanation

The Lewis electron dot structure for given molecules are determined by first drawing the skeletal structure for the given molecules, then the total number of valence electrons for all atoms present in the molecules are determined.

The next step is to subtract the electrons present in the total number of bonds present in the skeletal structure of the molecule with the total valence electrons such that considering each bond contains two electrons with it.

Finally, the electrons which got after subtractions have to be equally distributed considering each atom contains eight electrons in its valence shell.

The Lewis structure for the given molecules and ions are as follows,

The Lewis structure of the CO32- molecules identified as follows,

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