A figura a seguir mostra modelos de bola e vareta para os...

A figura a seguir mostra modelos de bola e vareta para os três formatos possíveis de uma molécula $\mathrm{AF}_3$. (a) Para cada estrutura, determine a geometria do domínio eletrônico na qual a geometria molecular é baseada. (b) Para cada estrutura, quantos domínios eletrônicos não ligantes há no átomo A? (c) Qual dos elementos a seguir produz uma molécula de $\mathrm{AF}_3$ com o mesmo formato de (ii): $\mathrm{Li}, \mathrm{B}, \mathrm{N}, \mathrm{Al}, \mathrm{P}, \mathrm{Cl}$ ? (d) Nomeie um elemento $\mathrm{A}$ o qual é esperado que conduza à estrutura $\mathrm{AF}_3$ mostrada em (iii). Explique seu raciocínio. FIGURE CANT COPY

Key Concepts

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) Theory is the foundational concept for predicting the three?dimensional arrangement of atoms in a molecule based on the repulsions between electron pairs in the valence shell of the central atom. By counting both bonding pairs and lone pairs and arranging them to minimize these repulsions, VSEPR theory helps determine the electron domain geometry, which in turn explains the observed molecular shape.

Electron Domain Geometry

Electron domain geometry refers to the arrangement of all electron groups—both bonding pairs and nonbonding (lone) pairs—around a central atom. The concept is based on the total number of electron domains (or regions of electron density), which dictates the idealized spatial arrangement aiming for maximum separation between these groups to reduce repulsion.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of only the atomic nuclei in a molecule, as influenced by the positions of the bonded atoms. Unlike electron domain geometry, molecular geometry omits the positions of lone pairs and emphasizes the shape formed solely by the bonded atoms, which is important in determining the chemical and physical properties of the molecule.

Bonding and Nonbonding Electron Pairs

Bonding electron pairs are those that participate in the formation of chemical bonds between the central atom and substituents, while nonbonding electron pairs (lone pairs) remain localized on the central atom. The presence of lone pairs affects the overall shape of the molecule by exerting a greater repulsive force than bonding pairs, thus distorting ideal bond angles and altering the molecular geometry.

Steric Number

The steric number is the sum of bonding pairs and lone pairs present on a central atom. This number is crucial for determining both the electron domain geometry and, when adjusted for the effects of lone pairs, the molecular geometry. A higher steric number indicates more regions of electron density and potentially more complex geometrical arrangements.

Periodic Trends and Electron Configuration

Periodic trends and electron configuration are important when predicting the formation and shape of molecules such as AF3. The number of valence electrons, the ability of atoms to expand their octet, and their electronegativity all play roles in determining whether a central atom will have lone pairs or an expanded bonding framework. Understanding these trends helps in identifying which elements can form molecules with particular geometries and electron domain configurations.

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