Consider the following XF₄ ions: PF₄^-, BrF₄^-, ClF₄⁺, and AlF₄^-. (b) For which of the ions will the electron-domain and molecular geometries be the same?

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Identify the central atom in each ion and determine the total number of valence electrons available for bonding. For example, phosphorus (P) in PF4- has 5 valence electrons, and the negative charge adds one more electron, totaling 6 valence electrons.

Determine the number of electron domains around the central atom. Each bond and lone pair counts as one domain. For instance, in PF4-, there are 4 bonds with fluorine atoms and no lone pairs, resulting in 4 electron domains.

Use the VSEPR (Valence Shell Electron Pair Repulsion) theory to predict the electron-domain geometry based on the number of electron domains. For 4 electron domains, the geometry is tetrahedral.

Determine the molecular geometry by considering only the positions of the atoms (ignoring lone pairs). If there are no lone pairs, the molecular geometry will be the same as the electron-domain geometry. For PF4-, with no lone pairs, the molecular geometry is also tetrahedral.

Repeat the process for each ion (BrF4-, ClF4+, and AlF4-) and compare the electron-domain and molecular geometries. Identify which ions have the same electron-domain and molecular geometries, indicating no lone pairs on the central atom.

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Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron domains (bonding and non-bonding pairs) around a central atom. It is determined by the number of electron domains, which can include lone pairs, single bonds, double bonds, and triple bonds. The geometry is predicted using the VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron domains will arrange themselves to minimize repulsion.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of the atoms in a molecule, excluding lone pairs. It is derived from the electron-domain geometry but focuses solely on the positions of the nuclei of the atoms. The molecular geometry can differ from the electron-domain geometry when there are lone pairs present, which can alter the shape of the molecule.

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs in the valence shell of the central atom. According to this theory, electron pairs will arrange themselves as far apart as possible to minimize repulsion, leading to specific molecular shapes. Understanding VSEPR is crucial for determining when electron-domain and molecular geometries will coincide, particularly in cases with no lone pairs.

Consider the following XF4 ions: PF4-, BrF4-, ClF4+, and AlF4-. (b) For which of the ions will the electron-domain and molecular geometries be the same?