The figure that follows shows ball-and-stick drawings of...

The figure that follows shows ball-and-stick drawings of three possible shapes of an $\mathrm{AF}_{3}$ molecule. (a) For each shape, give the electron-domain geometry on which the molecular geometry is based. (b) For each shape, how many nonbonding electron domains are there on atom $\mathrm{A}$ ? (c) Which of the following elements will lead to an $\mathrm{AF}_{3}$ molecule with the shape in (ii): $\mathrm{Li}, \mathrm{B}, \mathrm{N}, \mathrm{Al}, \mathrm{P}$, Cl? (d) Name an element A that is expected to lead to the $\mathrm{AF}_{3}$ structure shown in (iii). Explain your reasoning.

Key Concepts

Central Atom Electron Domain Count

The count of electron domains around a central atom—including both bonding domains (ligands) and nonbonding domains (lone pairs)—is critical for predicting molecular geometry. Understanding the central atom's total number of valence electrons allows one to determine how many bonds and lone pairs it will have, which in turn influences the specific shape of the molecule.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is the fundamental principle used to predict the geometry of molecules by considering the repulsions between electron pairs around a central atom. The theory posits that electron pairs, whether they are bonding or nonbonding, will arrange themselves as far apart as possible to minimize repulsive interactions, which in turn determines the overall electron-domain geometry of the molecule.

Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron density regions (bonding and nonbonding electron pairs) around a central atom. This concept is essential because it not only guides the shape of the molecule but also provides the framework on which molecular geometry is based. The electron-domain geometry is determined solely by the number of electron pairs present around the central atom.

Nonbonding Electron Domains

Nonbonding electron domains, commonly known as lone pairs, are pairs of valence electrons that are not involved in forming bonds between atoms. These lone pairs occupy space, affecting the molecular shape since they exert greater repulsive forces compared to bonding electron pairs. Their presence often leads to distortions in the ideal bond angles predicted by the electron-domain geometry.

Axial and Equatorial Positions

In molecules with geometries such as trigonal bipyramidal, the electron domains can occupy different positions (axial or equatorial) that are not equivalent in terms of repulsive interactions. Lone pairs tend to prefer the equatorial positions, where the repulsive interactions are minimized, thus playing a significant role in determining the observed molecular geometry as opposed to the simple electron-domain geometry.

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