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Chapter 10: Problem 5

In the trigonal bipyramidal arrangement, why does a lone pair occupy an equatorial position rather than an axial position?

Short Answer

Expert verified
A lone pair occupies an equatorial position rather than an axial position in a trigonal bipyramidal molecule because it minimizes the electron pair repulsion, being 90° away from just two other atoms instead of three in an axial position.

Step by step solution

01

Understand Trigonal Bipyramidal Arrangement

Firstly, understand that a trigonal bipyramidal arrangement is a molecular shape with one atom at the center and five atoms surrounding it. It has two types of positions that the surrounding atoms can occupy, axial and equatorial. The axial positions are aligned straight up and down from the center, while equatorial positions lie in the same horizontal plane as the center atom.
02

Explain Electron Pair Repulsion

The VSEPR theory postulates that electron pairs (either bonding or lone pairs) will orient themselves in such a way as to minimize repulsive interactions. In this context, it's important to understand that lone pair–lone pair repulsion is greater than lone pair–bonding pair repulsion, which is in turn greater than bonding pair–bonding pair repulsion. Essentially, lone pairs want to be as far away from each other as possible.
03

Explain Position Preference

In a trigonal bipyramidal arrangement, there are two axial positions and three equatorial positions. At an axial position, a lone pair would be 90° away from three atoms and directly opposite (180°) another atom. Whereas in an equatorial position, a lone pair would be 90° away from just two atoms and directly opposite (180°) from no atoms. Therefore, in order to minimize the repulsion, which is the aim according to VSEPR theory, a lone electron pair will always occupy an equatorial position when available instead of an axial position.

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