Valence-shell electron-pair repulsion concept

In many cases we can successfully predict the 3D structure of a molecule by using a very simple tool the Valence Shell Electron Pair Repulsion concept. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

Valence-shell electron-pair repulsion (VSEPR) theory and the concept of hybridization suggest that the water molecule has two O—H bonds and two non-bonded pairs arranged tetrahedrally. More accurate calculations show that this does not provide a true picture of the total electron density in H20. 

Organic chemists find that the information obtained from MO theory, where valence electrons occupy bonding and antibonding molecular orbitals, does not always yield the needed information about the bonds in a molecule. The valence-shell electron-pair repulsion (VSEPR) model combines the Lewis concept of shared electron pairs and lone-pair electrons with the concept of atomic orbitals and adds a third principle the minimization of electron repulsion. In this model, atoms share electrons by overlapping... 

A theory which is particularly useful in making predictions about the shapes of molecules is the valence-shell electron-pair repulsion (VSEPR) theory. The ideas behind this theory were first suggested in 1940 by the British chemists N. V. Sidgwick (1873-1952) and H. E. Powell. These concepts were later developed further by Sir Ronald Nyholm (1917-1971) and, more particularly, by the Canadian chemist Ronald... 

To predict the shapes of a variety of molecules we will use a theory which postulates that the shapes of molecules depend on the total number of bonded, and nonbonded or lone electron pairs surrounding a central atom. The chief concept is that electrons tend to repel each other, and that the mutual repulsion of all the electron pairs results in the molecules shape. The name of the method is therefore the valence-shell electron-pair repulsion theory, or the VSEPR theory. 

Skill 21.2 Apply the concepts of Lewis structures, valence-shell electron-pair repulsion, and hybridization to describe molecular geometry and bonding. 

Knowledge of the accurate electron density is decisive especially for the development of chemical concepts that are based on the analysis of this observable. Such concepts are Gillespie s valence-shell electron-pair repulsion model [1149] or the ligand-induced charge concentrations [880,1150-1152] that are designed to predict molecular structures and even chemical reactivity. Both approaches can be related to Bader s theory of atoms in molecules [1153], for which relativistic generalizations have been discussed in the literature [1154,1155]. 

The concept of valence-shell electron-pair repulsion (VSEPR) is presented in introductory organic chemistry as a way to predict molecular geometries. The idea behind VSEPR is that areas of electron density repel each other so that the geometry of bonds and/or lone pairs of electrons around any one atom places these areas as far apart as possible. Por four areas of electron density a tetrahedral geometry is predicted. Eor three areas of electron density a trigonal planar geometry is predicted. Two areas of electron density lead to a linear geometry. 

The power of the concept of localized orbital hybridization, combined with the simple ideas that (a) intemuclear repulsion of nonbonded nuclei should be minimized and (b) electron pairs filling orbitals also repel other filled orbitals (sometimes called valence shell electron pair repulsion [VSEPR]) is enormous. It allows us (1) to make predictions about the geometry of molecules or, (2) knowing the geometry, to make predictions about areas in space where electrons might be, and (3) to justify, to ourselves, why and how reactions might occur. The following examples are illustrative of these concepts ... 

The theoretical basis for a molecule possessing a particular geometric shape is the concept that electron pairs, whether they are part of a covalent bond (as in a bonding pair) or not (as in a nonbonding pair), will repel each other. In methane, water, and all other molecules, this repulsion means that the electron pairs will get as far away from each other as they can get. The general name for this theory is the valence-shell electron-pair repulsion (VSEPR) theory. 

Organic chemists led the way in picturing molecular bonding. They relied on such concepts as radicals (which kept their identity through various reactions) and atoms with a fixed valence or combining power. Once the electron was discovered in the early part of the twentieth century, Lewis was able to explain some aspects of bonding on the basis of his electron-dot formulas and the octet mle. The valence-shell electron-pair repulsion (VSEPR), valence-bond (VB), and molecular orbital (MO) theories followed in the 1930s. 

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