   Chapter 8.6, Problem 8.9CYU

Chapter
Section
Textbook Problem

Give the electron-pair geometry and molecular shape for BF3 and BF4‒ What is the effect on the molecular geometry of adding an F– ion to BF3 to give BF4‒?

Interpretation Introduction

Interpretation:

The electron pair geometry and molecular shape for BF3andBF4- should be determined and effect on molecular geometry of adding F- to BF3 that forms BF4- should be identified.

Concept Introduction:

Molecular geometry: It is defined as unique three dimensional arrangements of atoms around the central metal present in the molecule which is determined by using spectroscopic techniques and also by using Lewis structure or the valence shell electron pair repulsion theory (VSEPR).

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.

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