Chapter 8, Problem 9STP

Interpretation Introduction

Interpretation: The arrangement of electron pairs in ${\text{PBr}}_{\text{3}}$ molecule needs to be explained. The molecular shape of the molecule needs to be predicted.

Concept Introduction: An electron dot structure represents the arrangement of total valence electrons in a molecule. Here, electrons are represented as dots in pairs to show the lone pairs as well as the bond pairs.

Explanation of Solution

In ${\text{PBr}}_{\text{3}}$ molecule, three bromine atoms are bonded with one phosphorus atom via single bonds. Here, the number of valence electrons of P is 5 and that of bromine is 7.

Since bromine is bonded with phosphorus via a single bond, the structure can be represented as follows:

  

There are 3 bonding pairs of electrons and 1 non-bonding pair; thus, the total number of electron pairs will be 4.

According to VSEPR theory, the following table shows the relation between the total number of bonding and non-bonding pair of electrons and the geometry of a molecule.

Total number of bonding and non-bonding pairs	Geometry
6	Octahedral
5	Trigonal pyramidal
4	Tetrahedral
3	Trigonal planar
2	Linear

Thus, the arrangement of the electron pair will be tetrahedral as per the VSEPR theory.

Due to lone pair-bond pair repulsion, the shape will be pyramidal which is similar to ${\text{NCl}}_{\text{3}}$ .

Therefore, the arrangement of the electron pair in ${\text{PBr}}_{\text{3}}$ molecule is tetrahedral and the molecular shape of the molecule is pyramidal.