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): $\mathrm{Li}, \mathrm{B}, \mathrm{N}, \mathrm{Al}, \mathrm{P}, \mathrm{Cl} ?(\mathbf{d})$ Name an element $\mathrm{A}$ that is $\mathrm{ex}-$ pected to lead to the $\mathrm{AF}_{3}$ structure shown in (iii).

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): $\mathrm{Li}, \mathrm{B}, \mathrm{N}, \mathrm{Al}, \mathrm{P}, \mathrm{Cl} ?(\mathbf{d})$ Name an element $\mathrm{A}$ that is $\mathrm{ex}-$
pected to lead to the $\mathrm{AF}_{3}$ structure shown in (iii).

Key Concepts

Valence Shell Electron Pair Repulsion (VSEPR) Theory

This concept explains how electron pairs (both bonding and nonbonding) around a central atom repel one another and arrange themselves as far apart as possible in three-dimensional space. The theory provides the basis for predicting the electron-domain geometry of a molecule, which in turn informs the observed molecular shape when nonbonding electrons are taken into account.

Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron groups (bonding pairs and lone pairs) around a central atom. It is determined by the total number of electron domains and is a fundamental concept in VSEPR theory, establishing the framework (such as tetrahedral, trigonal bipyramidal, or octahedral) upon which the observed, sometimes distorted, molecular geometry is based.

Nonbonding Electron Domains (Lone Pairs)

Nonbonding electron domains, or lone pairs, are regions of electron density that are not involved in bonding. Their presence influences the molecular shape by exerting repulsive forces, which can alter bond angles and lead to deviations from the ideal geometry predicted by simply considering the bonding pairs.

Lewis Structures and Electron Counting

Lewis structures are visual representations of the valence electron arrangement within a molecule. Electron counting, an integral part of drawing these structures, helps determine the number of bonding and nonbonding electron pairs around the central atom. This process is crucial for predicting both the electron-domain geometry and the resulting molecular geometry.

Periodic Trends in Valence Electrons

Understanding periodic trends in the number of valence electrons is key when predicting molecular structures. The central atom’s position in the periodic table provides insights into its bonding capacity and the likelihood of retaining lone pairs. This information, combined with VSEPR theory, helps in anticipating molecular shapes, as different electron counts can lead to various arrangements of bonding and nonbonding electron domains.