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 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 A? (c) Which of the following elements will lead to an $\mathrm{AF}_{3}$ molecule with the shape in (ii): Li, B, N, Al, P, Cl? (d) Name an element A that is expected to lead to the $\mathrm{AF}_{3}$ structure shown in (iii).

Key Concepts

Steric Number and Valence Electron Counting

The steric number, which is determined by counting both bonding pairs and lone pairs around the central atom, is essential in predicting the electron-domain geometry of a molecule. By counting the valence electrons and assigning them to either bonding or nonbonding domains, chemists can deduce the overall geometric arrangement and understand how different elements may lead to different structures in compounds such as AF3.

Molecular Geometry

Molecular geometry describes the shape of a molecule as determined by the positions of the atoms, excluding the lone pairs. While the electron-domain geometry includes all electron groups, the molecular geometry only takes into account the bonding pairs, which results in observable arrangements such as linear, trigonal planar, tetrahedral, etc., modified by the presence of nonbonding electron domains.

Nonbonding Electron Domains (Lone Pairs)

Nonbonding electron domains, commonly known as lone pairs, are pairs of valence electrons that do not participate in bonding. Their presence on the central atom affects the shape of the molecule by exerting additional repulsive forces, which can lead to distortions from the ideal electron-domain geometry resulting in a different molecular geometry.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the three?dimensional arrangement of atoms in a molecule based on the number of electron domains (bonding and nonbonding pairs) around the central atom. It assumes that electron pairs repel each other and thus adopt positions that minimize repulsion, ultimately defining both the electron-domain and molecular geometries.

Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron groups (both bonding pairs and lone pairs) surrounding a central atom. This concept is crucial because it represents the ideal geometry that would be observed if all electron pairs (including nonbonding electron domains) were equally visible, which then influences the observed molecular (or actual) geometry when lone pairs are present.

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