Chapter MS, Problem 8PQ

Interpretation Introduction

Interpretation:

The shape of ${\text{XeF}}_{\text{4}}$ molecule has to be determined.

Concept Introduction:

VSEPR or valence shell electron pair repulsion theory predicts shape of molecules with the 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 atoms.

2. A distortion is observed in shape of 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 to different number of electron pairs:

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

The geometry of the molecule is predicted with the help of steric number. The formula to calculate the 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)

The least electronegative atom is usually taken to be central atom.