VSEPR Model of Chemical Bond

There is history on how Newton G. Lewis had introduced the doublet and octet electronic mle in the valence layer by the cubic phenomenology, see Chapter 3 of the Volume IV of the present five-volume work (Putz, 2016b). However, even if the quantum theory that follows had invalidated the cubic atom paradigm, the rationalization of the chemical bond through the electrons doublet or of the electrons pair (with associated spins) survived all the orbital approaches, being definitively consecrated by the Pauli Exclusion Principle. 

On the other side, as abundantly shown in crystal chemistry, see Voliune IV of the present five-volume work (Putz, 2016b), the constitutive particles representation of a crystal is taking the form of a rigid spheres with a characteristic and constant dimension (at least for a certain stmcture imder concern) directly leads to the conclusion of their arrangement way in crystalline lattice, resulting that the symmetry of elementary cell and the crystalline reason will be conditioned by geometric factors. 

the positional relations established in a stmcture, between a particle and all the other generically define the neighborhoodparadigm of the considered particle, geometrizing the notion of chemical bond. 

The closest neighbors, i.e., the particles towards lead the shortest vectors form the particles of the first sphere of coordination, while the following neighbors - of the second sphere and so on. 

If the sphere includes m particles (i.e., on the sphere with radius eqital with the module of the nth bondirrg vector are found m particles) is considered that the particle in cause has inside the n sphere the coordination number m. 

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What advantages does the VSEPR model of chemical bonding have compared with Lewis formulas ... 

In the models of chemical bonding we have discussed up to now, we have assumed that the electrons that interpose themselves between adjacent nuclei (the bonding electrons ) are in orbitals associated with one or the other of the parent atoms. In the simple Lewis and VSEPR models, these were just the ordinary s, p, and d orbitals. The more sophisticated hybridization model recognized that these orbitals will be modified by the interaction with other atoms, and the concept of mixed (hybrid) orbitals was introduced. 

We first examine the role of chemical bonds and lone pairs on the geometry of a molecule in terms of a simple approach called the VSEPR model. (10.1)... 

The VSEPR model, based largely on Lewis structures, provides a relatively simple and straightforward method for predicting the geometry of molecules. But as we noted earlier, the Lewis theory of chemical bonding does not clearly explain why chemical bonds exist. Relating the formation of a covalent bond to the pairing of electrons was... 

As much as the Lewis and VSEPR models help us to understand covalent bonding and the geometry of molecules, they leave many questions unanswered. The most important of these questions is the relation between molecular structure and chemical reactivity. For example, carbon-carbon double bonds are different in chemical reactivity from carbon-carbon single bonds. Most carbon-carbon single bonds are quite unreactive but carbon-carbon double bonds, as we will see in Chapter 5, react with a wide variety of reagents. The Lewis model and VSEPR give us no way to account for these differences. Therefore, let us turn to a newer model of covalent bonding, namely, the formation of covalent bonds by the overlap of atomic orbitals. 

The concept of local charge accumulation and depletion, analyzed by the negative Laplacian of the electron density distribution, offers much more than a description of chemical bond properties and the location of non-bonded pairs of electrons, although this is a sound basis for the VSEPR model. Regions of local charge... 

Presents in an unitary quantum theory the various aspects of chemical bonding, from classical description of valence bond and molecular orbital theories to the modem bondonic-bosonic picture of chemical field and interaction, to crystal and ligand field related with symmetry properties and eigen-groups of molecules, to Valence Shell Electron Pair Repulsion-VSEPR model ... 

Lewis s original theory could not take into account the shape adopted by molecules. Gillespie and Nyhohn (1957) developed the currently accepted modem theory of chemical bond formation (MO and VB theories), which uses the valence-shell electron pair repulsion model (VSEPR) to account for molecular structure (Gillespie, 1970). VSEPR states that molecular shape is caused by repulsions between electron pairs in the valence shell. 

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],... 

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