Question

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

Short answer

A lone pair occupies an equatorial position in a trigonal bipyramidal arrangement to minimize repulsion with other electron pairs, because the equatorial position is further away from the other electron pairs.

Step-by-step solution

Understanding the Trigonal Bipyramidal Structure

A molecule with a trigonal bipyramidal structure has one central atom and five surrounding atoms. Imagine this arrangement as a central atom with three equatorial atoms forming a triangle around it at the base, while two additional axial atoms are perpendicular to the equatorial atoms forming a line that goes through the central atom.

Understanding Lone Pair Positioning

In a molecule, lone pair electrons are concentrated in one region, creating a high negative charge density. This dense region repels other electron pairs in the molecule. The repulsion between two electron pairs decreases with increasing distance

Explaining the Position in Trigonal Bipyramidal Arrangement

In a trigonal bipyramidal structure, a lone pair of electrons on the central atom will be placed in the equatorial position. This is due to the fact that equatorial positions are 120° away from two other electron pairs and 90 ° away from other two, hence repulsions are minimized. If a lone pair occupied an axial position, it would be 90° away from three other electron pairs, leading to increased repulsion.