Valence-shell electron-pair repulsion VSEPR method

This method for determining molecular shape by minimizing electfon repulsion is called the valence-shell electron-pair repulsion (VSEPR) method. Note that we often draw molecules such as BCI3 and CH4 as if they were flat and had 90° angles. This depiction is for... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

Like so many other molecular properties, shape is determined by the electronic structure of the bonded atoms. The approximate shape of a molecule can often be predicted by using what is called the valence-shell electron-pair repulsion (VSEPR) model. Electrons in bonds and in lone pairs can be thought of as "charge clouds" that repel one another and stay as far apart as possible, thus causing molecules to assume specific shapes. There are only two steps to remember in applying the VSEPR method ... 

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of the discussion of bonding in this book uses the molecular orbital approach to chemical bonding, simpler methods that provide approximate pictures of the overall shapes and polarities of molecules are also very useful. This chapter provides an overview of Lewis dot structures, valence shell electron pair repulsion (VSEPR), and related topics. The molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters, but the ideas of this chapter provide a starting point for that more modem treatment. General chemistry texts include discussions of most of these topics this chapter provides a review for those who have not used them recently. 

Many experimental methods now exist for determining the molecular stmcture of a molecule—that is, the three-dimensional arrangement of the atoms. These methods must be used when accurate information about the stmcture is required. However, it is often useful to be able to predict the approximate molecular structure of a molecule. Now we will consider a simple model that allows us to do this. The valence shell electron pair repulsion (VSEPR) modei is useful for predicting the molecular structures of molecules formed from nonmetals. The main idea of this model is that... 

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. 

VSEPR (valence-shell electron-pair repulsion) A method of predicting the shape of molecules. In this theory one atom is taken to be the central atom and pairs of valence electrons are drawn round the central atom. The shape of the molecule is deter-... 

Valence shell electron-pair repulsion (VSEPR) is an approach that provides a method for predicting the shape of molecules based on the electron-pair electrostatic repulsion described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957 and in the succeeding decades. Despite this method s simple approach, based on Lewis electron-dot strnctnres, the VSEPR method in most cases predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules. The most common method of determining the actual structures is X-ray diffraction, although electron diffraction, nentron diffraction, and many spectroscopic methods are also used. In Chapter 5, we will provide molecular orbital approaches to describe bonding in simple molecules. 

Unrestricted Hartree-Fock (UHF) method (p. 408) Valence Shell Electron Pair Repulsion (VSEPR) (p. 49l) variation of a spinorbital (p. 401) virtual orbital (p. 410)... 

Valence Shell Electron Pair Repulsion (VSEPR), 491 van der Waals interaction energy, 347, 796 van der Waals radius, 849 variable. Boolean, 382 variational function, 399 variational method, 838, 940... 

The bonding in ground state PHg was also described in the framework of a generalized valence bond (GVB) theory [17]. A VB method, which was combined with the method of atoms in molecules [18], yielded a small d contribution (4%) in the P valence state [19]. For an application of the valence shell electron pair repulsion (VSEPR) theory, see [20]. 

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. 

The valence-shell electron-pair repulsion (VSEPR) rule states that aU groups emanating from an atom—whether single, double, or triple bonds, or lone pairs— wiU be in spatial positions that are as far apart from one another as possible. The VSEPR method does not consider singly occupied orbitals to be groups (see below for the reason). VSEPR is purely a theory based upon the notion that the electrostatic repulsions between entities consisting of two or more electrons dictate molecular geometries. 

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