Valence electrons VSEPR theory

The other approach to molecular geometry is the valence shell electron-pair repulsion (VSEPR) theory. This theory holds that... 

The shape of a molecule has quite a bit to do with its reactivity. This is especially true in biochemical processes, where slight changes in shape in three-dimensional space might make a certain molecule inactive or cause an adverse side effect. One way to predict the shape of molecules is the valence-shell electron-pair repulsion (VSEPR) theory. The... 

VSEPR theory The VSEPR (valence shell electron-pair repulsion) theory says that the electron pairs around a central atom will try to get as far as possible from each other in order to minimize the repulsive forces. This theory is used to predict molecular geometry. 

Introducing Valence-Shell Electron-Pair Repulsion (VSEPR) Theory... 

As can be seen from Table I, these weak CIF bonds occur only when the central atom has a coordination number in excess of 4 and possesses at least one free Cl valence electron pair. In addition to Gillespie s simple VSEPR theory, the following general rule has been proposed by Christe (53), which permits the prediction of whether, and how many, semi-ionic bonds are to be formed ... 

The basic principle underlying VSEPR theory is that valence electron pairs, whether they re lone pairs or they occur within bonds, prefer to be as far from one another as possible. There s no sense in crowding negative charges any more than necessary. 

Consider one beautifully symmetrical shape predicted by VSEPR theory the tetrahedron. Four equivalent pairs of electrons in the valence shell of an atom should distribute themselves into such a shape, with equal angles and an equal distance between each pair. But what sort of atom has four equivalent electron pairs in its valence shell Aren t valence electrons distributed between different kinds of orbitals, like s and p orbitals (We introduce these orbitals in Chapter 4.)... 

The basis of the VSEPR theory is that the shape of a molecule (or the geometry around any particular atom connected to at least two other atoms) is assumed to be dependent upon the minimization of the repulsive forces operating between the pairs of sigma (a) valence electrons. This is an important restriction. Any pi (7t) or delta (8) pairs are discounted in arriving at a decision about the molecular shape. The terms sigma , pi and delta refer to the type of overlap undertaken by the contributory atomic orbitals in producing the molecular orbitals, and are referred to by their Greek-letter symbols in the remainder of the book. 

The VSEPR theory assumes that the four electrons from the valence shell of the carbon atom plus the valency electrons from the four hydrogen atoms form four identical electron pairs which, at minimum repulsion, give the observed tetrahedral shape. To rationalize the tetrahedral disposition of four bond-pair orbitals with those of the 2s and three 2p atomic orbitals of the carbon atom, sp3 hybridization is invoked. 

The predictions of molecular shape by VSEPR theory are summarized in Figure 6.17 for those molecules which have no n bonding. To the left side of Figure 6.17 are the basic shapes adopted by molecules which are covalently saturated, i.e. all the valence electrons are used in bond for-... 

Vacancy mechanism, 266 Valence bond (VB) theory, 139-153. 391-394.474 Valence shell electron pair repulsion (VSEPR) model. 203-206. 217-218... 

The perfectly octahedral species conform to the expectations based on the simple MO derivation given above. The nonoctahedral fluoride species do not, but this difficulty is a result of the oversimplifications in the method. There is no inherent necessity for delocalized MOs to be restricted to octahedral symmetry. Furthermore, it is possible to transform delocalized molecular orbitals into localized molecular orbitals. Although the VSEPR theory is often couched in valence bond terms, it depends basically on the repulsion of electrons of like spins, and if these are in localized orbitals the results should be comparable. 

The most useful approach in predicting the stereochemistry of coordination compounds is the Valence Shell Electron Pair Repulsion theory (VSEPR theory), which is based on the idea that the electron pairs around an atom will be arranged so as to minimize the repulsion between them. The qualitative foundations of this field were laid by Sidgwick and Powell1 in 1940 and by Gillespie... 

The electron-dot structures described in Sections 7.6 and 7.7 provide a simple way to predict the distribution of valence electrons in a molecule, and the VSEPR model discussed in Section 7.9 provides a simple way to predict molecular shapes. Neither model, however, says anything about the detailed electronic nature of covalent bonds. To describe bonding, a quantum mechanical model called valence bond theory has been developed. 

An advantage of VSEPR is its foundation upon Lewis electron-pair bond theory. No mention need be made of orbitals and overlap. If you can write down a Lewis structure for the molecule or polyatomic ion in question, with all valence electrons accounted for in bonding or nonbonding pairs, there should be no difficulty in arriving at the VSEPR prediction of its likely shape. Even when there may be some ambiguity as to the most appropriate Lewis structure, the VSEPR approach leads to the same result. For example, the molecule HIO, could be rendered, in terms of Lewis theory as ... 

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