Chapter 9, Problem 1KSP

Interpretation Introduction

Interpretation:

The molecular geometry of ${\text{PBr}}_{\text{3}}$ is to be determined.

Concept introduction: In order to find the geometry of a molecule, follow the steps:

First, draw the Lewis dot structure of the molecule.

Find the number of electron pairs or domains around the central atom. Electron domain is the bond and lone pair of electrons around the central atom.

Use the VSEPR model to find the electron domain geometry.

According to VSEPR, the electron domains repel each other and arrange themselves as apart as possible in space.

Molecular geometry is the arrangement of bonded atoms. If no lone pairs are present, the molecular geometry is the same as the electron domain geometry.

The repulsion between two lone pairs is greater than that in case of lone pair–bond pair, which, in turn, is greater than the bond pair–bond pair repulsion.

Answer to Problem 1KSP

Correct answer: Option (c)

Explanation of Solution

Reason for the correct option:

Lewis dot structure of ${\text{PBr}}_{\text{3}}$ is shown below:

In this molecule, the phosphorus atom is bonded to three bromine atoms by single bonds and there is one lone pair on the phosphorus atom.

Thus, the electron domains around the phosphorus atom are four.

According to the VSEPR model, the four electron domains repel each other and arrange themselves in space in a tetrahedral shape.

However, in this case, one lone pair is present on the phosphorus atom.

Thus, the molecular geometry is trigonal pyramidal.

Hence, option (c) is correct.

Reasons for the incorrect options:

Option (a) is incorrect because a trigonal planar shape is obtained when there is no lone pair on the central atom.

Option (b) is incorrect because a tetrahedral shape is obtained when four bonds are present.

Option (d) is incorrect because a bent shape is obtained when there are two bonds present.

Option (e) is incorrect because a T-shape is obtained from the trigonal bipyramidal geometry with two lone pairs.

Hence, options (a), (b), (d), and (e) are incorrect.