Valence shell electron pair domain

Figure 4.6 Styrofoam sphere models representing the arrangements of two, three, four, five, and six valence shell electron pair domains.

The VSEPR model is an approach whereby bonding pairs and lone pairs are considered as electron domains. These domains interact with each other through an exclusion process (according to Gillespie, one of the original developers of the VSEPR model, the model should be more properly called as valence shell electron pair domain, as they do not interact as a repulsive process [13]). These exclusions will then give rise to different geometries for various compounds. 

Once computed on a 3D grid from a given ab initio wave function, the ELF function can be partitioned into an intuitive chemical scheme [30], Indeed, core regions, denoted C(X), can be determined for any atom, as well as valence regions associated to lone pairs, denoted V(X), and to chemical bonds (V(X,Y)). These ELF regions, the so-called basins (denoted 2), match closely the domains of Gillespie s VSEPR (Valence Shell Electron Pair Repulsion) model. Details about the ELF function and its applications can be found in a recent review paper [31],... 

It is a small step from van der Waals, electron-domain models of the C—H bonds of, e.g., biphenyl, cyclohexane, or methane (Figs. 1—3), to molecular models in which to a first, and useful, approximation each valence-shell electron-pair is represented by a spherical, van der Waals-like domain 7h (Non-spherical domains may be useful for describing, e.g., lone pairs about atoms with large atomic cores, -electrons, and the electron-pairs of multiple bonds vide infra.)... 

Fig. 4 is a drawing of an all-valence-shell-electron-domain model of ethane superimposed on the molecule s conventional graphic formula. Not shown are the electron-domains of the carbon atoms Is electrons. In Fig. 4, each valence-stroke, i.e. each valence-shell electron-pair of ethane, protonated ("C—H") or unprotonated ( C—C"), is represented by a van der Waals sphere. 

Valence Shell Electron Pair Repulsion. Both bonding pairs of electrons and nonbonding pairs are represented by EP. Both act as negative electron domains. 

This theory is called valence-shell electron-pair repulsion (VSEPR) theory or electron-domain (ED) theory . It is described in detail in most textbooks. 

The Group 13 atom in a trichloride or a trimethyl derivative is surrounded by three bonding electron pairs. According to the VSEPR model the three valence shell electron pairs surrounding the metal atom repel one another and occupy domains near the comers of an equilateral triangle with the metal atom at the center. The observed trigonal planar structures are thus in accord with the VSEPR model, but the model provides no explanation for the observation that the bonds in the trichlorides are shorter and weaker than in the monochlorides. 

THE VSEPR MODEL (SECTION 9.2) The valence-shell electron-pair repulsion (VSEPR) model rationalizes molecular geometries based on the repulsions between electron domains, which are regions about a central atom in which electrons are likely to be found. Bonding pairs of electrons, which are those involved in making bonds, and nonbonding pairs of electrons, also called lone pairs, both create electron domains aroimd an atom. According to the VSEPR model, electron domains orient themselves to minimize electrostatic repulsions that is, they remain as far apart as possible. 

According to the valence-shell electron-pair repulsion (VSEPR) model, electron pairs in the valence shell of an atom repel one another. An electron domain is a lone pair or a bond. Any bond (single, double, or triple) constitutes one electron domain. 

Bond angle, 342 Electron-domain geometry, 342 Molecular orbital theory, 368 Valence-shell electron-pair... 

According to the Valence Shell Electron Pair Repulsion model (VSEPR) electron domains spread out as far as possible from one another (repel one another). 

Two, Three, Four, and Six Electron Pair Domain Valence Shells 93... 

We discuss molecules with a valence shell containing five electron pair domains in Section 4.6. The preferred arrangements of five valence shell domains, the trigonal bipyramid and the square pyramid, are not regular polyhedra and therefore exhibit special features not found in tetrahedral and octahedral molecules. Molecules with seven and more electron pair domains in the valence shell of a central atom are not common, although they are of considerable interest. They are restricted to the elements of period 4 and higher periods, with very small ligands such as fluorine, and are discussed in Chapter 9. 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

Electrons in the core of an atom are fully localized into spherical shells but not into opposite-spin pairs. In an isolated atom the valence shell electrons are similarly localized into a spherical shell. The Laplacian shows that in each of these spherical shells there is a spherical region of charge concentration and a spherical region of charge depletion. But in these regions there is no localization of electrons of opposite spin into pairs. There are no Lewis pairs or electron pair domains in an inner shell. The domain of each electron is spherical and fully delocalized through the shell. 

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