Chapter MS, Problem 17PQ

Interpretation Introduction

Interpretation:

The conclusion from fact that ${\text{BF}}_{\text{3}}$ has no dipole moment but ${\text{PF}}_{\text{3}}$ has dipole moment has to be determined.

Concept Introduction:

Valence shell electron pair repulsion theory (VSEPR) predicts shape of molecules with use of arrangement of electrons around central metal atom. Its postulates are as follows:

1. The shape of molecules is governed by repulsion between electron pairs situated in the outermost or valence shell of the atoms.

2. A distortion is observed in the shape of the molecules if lone pairs are present in it. These are electron pairs that do not participate in the chemical bonding between the atoms. The electron pairs that take part in bond formation are known as bond pairs.

3. The order of repulsion of electron pairs is as follows:

  $\text{lp}-\text{lp repulsion}>\text{lp}-\text{bp repulsion }>\text{bp}-\text{bp repulsion}$

4. Multiple bonds create more repulsion as compared to single bonds.

5. The following table shows various molecular shapes that match different number of electron pairs:

$\begin{array}{ccc}\begin{array}{l}\text{Steric number or}\\ \text{number of}\\ \text{electron groups}\end{array}& \begin{array}{l}\text{Number of }\\ \text{lone pairs}\end{array}& \text{Molecular geometry}\\ 2& 0& \text{Linear}\\ 3& 0& \text{Trigonal planar}\\ 3& 1& \text{Bent}\\ 4& 0& \text{Tetrahedral}\\ 4& 1& \text{Trigonal pyramidal}\\ 4& 2& \text{Bent}\\ 5& 0& \text{Trigonal bipyramidal}\\ 5& 1& \text{See}-\text{saw}\\ 5& 2& \text{T}-\text{structure}\\ 5& 3& \text{Linear}\\ 6& 0& \text{Octahedral}\\ 6& 1& \text{Square pyramidal}\\ 6& 2& \text{Square planar}\end{array}$

The geometry of the molecule is predicted with help of the steric number. The formula to calculate steric number is as follows:

  $\text{Steric number}=\left[\begin{array}{c}\left(\text{Atoms bonded to the central atom}\right)+\\ \left(\text{Number of lone pairs on the central atom}\right)\end{array}\right]$        (1)

Bond angle is angle that exists between two bonded electron pairs or simply between two bonds. It is the angle that is formed between three atoms with atleast two bonds. It is a major contributor to shape of molecule and is helpful to predict the difference between various geometries of molecules.