Valence-shell electron-pair basis

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

In some respects arenediazonium ions show analogies to acetylene. Acetylene has two deformation vibrations, v4 at 613.5 cm-1 and v6 at 729.6 cm-1, as shown in Figure 7-1 (Feldmann et al., 1956). The fact that the symmetrical vibration v4 has a lower frequency than v6 can be understood from BartelPs valence-shell electron-pair repulsion (VSEPR) model (1968) on the basis of a <pseudo-Jahn-Teller> effect. 

The Laplacian of the electron density plays a dominant role throughout the theory.191 In addition, Bader has shown that the topology of the Laplacian recovers the Lewis model of the electron pair, a model that is not evident in the topology of the electron density itself. The Laplacian of the density thus provides a physical valence-shell electron pair repulsion (VSEPR) basis for the model of molecular geometry and for the prediction of the reaction sites and their relative alignment in acid-base reactions. This work is closely tied to earlier studies by Bader of the electron pair density, demonstrating that the spatial localization of electrons is a result of a corresponding localization of the Fermi correlation hole. 

The structures of the binary fluorides are predictable on the basis of the valence shell electron pair repulsion model (see Chapter 2). With eight valence shell electrons from the xenon atom and two additional electrons from the two fluorine atoms, there are 10 electrons surrounding the xenon atom in XeF2. Thus, the structure of XeF2 has Doah symmetry as shown here ... 

A. Schmiedekamp, D. W. J. Cruickshank, S. Skaarup, P. Pulay, I. Hargittai, J. E. Boggs, Investigation of the Basis of the Valence Shell Electron Pair Repulsion Model by ab Initio Calculation of Geometry Variations in a Series of Tetrahedral and Related Molecules. J. Am. Chem. Soc. 1979, 101, 2002-2010. 

The geometric structure of the covalent binary halides, whether neutral or complexed ions, can be explained on the basis of the Nyhotm-Gillespie rules known as the Valence Shell Electron Pair Repulsion Model (VSEPR) theory the geometrical arrangements of the bonds around an atom in a species depends on the total number of electron pairs in the valence shell of the central atom, including both bonding... 

Background Covalent bonding occurs when atoms share valence electrons. In the Valence Shell Electron Pair Repulsion (VSEPR) theory, the way in which valence electrons of bonding atoms are positioned is the basis for predicting a molecule s shape. This method of visualizing shape is also based on the molecule s Lewis structure. 

In this section, we will study the valence shell electron pair repulsion theory and the basis for predicting the shapes of some structures. 

Schmiedekamp A, Cruickshank DWJ, Skaamp S, Pulay P, Haigittai I, Boggs JE (1979) Investigation of the basis of the valence shell electron pair repulsion model by ab initio calculation of geometry variations in a series of tetrahedral and related molecules. J Am Chem Soc 101 2002-2010... 

We can predict the arrangement of atoms in moiecuies and ions on the basis of a reiativeiy simpie idea caiied the valence shell electron pair repulsion (VSEPR) model. 

Finally, in addition to simply representing a pair of shared electrons, a chemical bond has structural implications as well. Because electrons are negatively charged, when there are several distinct bonds, they will tend to be physically separated from each other. This idea is the basis for a method to predict the geometry of molecules called the Valence Shell Electron Pair Repulsion (VSEPR) theory. Using this theory, the general shape of molecules and ions can be predicted. 

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