Shell Electron Pair Repulsion Theory

Valence Shell Electron Pair Repulsion Theory 

Valence shell electron pair repulsion theory (VSEPR) can be used to predict the shapes of molecules. According to this theory, the geometry of a molecule is such that the valence-electron pairs of the central atom are kept farthest apart to minimize the electron repulsions. Again, you have to view molecules in terms of Lewis structure so that the shape of the molecules can be predicted with the VSEPR theory. 

Molecular geometry of a molecule is the directional orientation of the bonded pairs aroimd the central atom, excluding the unshared electron pairs. 

Let s look at the practical significance of this theory through some examples. 

Carbon, obviously the central atom, has four electrons in its valence shell. In order to complete the octet, carbon requires four more electrons or two pairs of electrons. Oxygen atom has six electrons in its valence shell. In order to complete its octet, each oxygen atom requires two electrons or one pair of electrons. The Lewis structure of COj should look like the figure shown below. Note that the valence electrons of the carbon atom are denoted by asterisks ( ) and that of the oxygen are denoted by dots. 

We have seen that the tetrahedral shape of methane is consistent with a bonding model based on sp hybrid orbitals on carbon. We should not conclude, however, that the geometry is a result of sp hybridization. We 

Pauling argued that the photoelectron spectrum of methane is consistent with sp hybridization for methane. Pauling, L. /. Chem. Educ. 1992,69,519. See also the discussion by Simons, ]. J. Chem. Educ. 1992, 69, 522. 

C-Cl bond length is reasonable, the H-C-H bond angle is greater than 109.5°, and the H-C-Cl angle is smaller than 109.5°. 

We can rationalize the difference between prediction and experiment with VSEPR theory by noting that the electronegativity of chlorine is greater than that of carbon (Table 1.12). In a methane C-H bond, the carbon atom and the hydrogen atom attract the electron pair approximately equally. In CH3CI, however, the electrons in the C-Cl bond will be pulled toward the 

The phrase valence shell electron pair repulsion, or VSEPR, sounds like a mouthful, but it is actually fairly simple to understand. Valence shell means we are only considering the valence electrons of an atom electron pair means that in a molecule, the electrons are always paired. Repulsion is the key term. Electrons are all negatively charged. VSEPR states that the electron pairs will move into relative orientation so as to maximize the distance the electron pairs are from each other. 

The atoms in molecules move into relative positions to maximize the distances between their valence electrons because that arrangement gives the configuration of atoms with the lowest 

Consider each example in Table 20.1. In the case of methane (CH ), there are foiu- pairs of electrons around the carbon atom (necessarily so in order to satisfy the octet rule). What geometrical arrangement can those four pairs of electrons assume that will maximize their distance apart from each other  

At first, you might be tempted to say that the pairs of electrons will assume positions at the four corners of a square. In a square, however, the angles between bonds would be only 90°. In addition, a square is flat, and many molecules are three-dimensional. Therefore, a better answer to the question is that the four pairs of electrons can assume positions at the vertices of a tetrahedron. 

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Valence shell electron pair repulsion theory, 1,32-39 effective bond length ratios, 1.34 halogenium species, 3, 312 noble gas compounds, 3,312 repulsion energy coefficient, 1, 33 Valency... 

Now that we know how to determine hybridization states, we need to know the geometry of each of the three hybridization states. One simple theory explains it all. This theory is called the valence shell electron pair repulsion theory (VSEPR). Stated simply, all orbitals containing electrons in the outermost shell (the valence shell) want to get as far apart from each other as possible. This one simple idea is all you need to predict the geometry around an atom. First, let s apply the theory to the three types of hybridized orbitals. 

In one respect the valence shell electron-pair repulsion theory is no better (and no worse) than other theories of molecular structure. Predictions can only be made when the constitution is known, i.e. when it is already known which and how many atoms are joined... 

To derive the values of the coefficients at, Ph y, and 8i so that the bond energy is maximized and the correct molecular structure results, the mutual interactions between the electrons have to be considered. This requires a great deal of computational expenditure. However, in a qualitative manner the interactions can be estimated rather well that is exactly what the valence shell electron-pair repulsion theory accomplishes. 

Redress can be obtained by the electron localization function (ELF). It decomposes the electron density spatially into regions that correspond to the notion of electron pairs, and its results are compatible with the valence shell electron-pair repulsion theory. An electron has a certain electron density p, (x, y, z) at a site x, y, z this can be calculated with quantum mechanics. Take a small, spherical volume element AV around this site. The product nY(x, y, z) = p, (x, y, z)AV corresponds to the number of electrons in this volume element. For a given number of electrons the size of the sphere AV adapts itself to the electron density. For this given number of electrons one can calculate the probability w(x, y, z) of finding a second electron with the same spin within this very volume element. According to the Pauli principle this electron must belong to another electron pair. The electron localization function is defined with the aid of this probability ... 

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. 

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. 

Gibson model, 38 174-176 Gillespie-Nyholm valence shell electron-pair repulsion theory, 18 325 Glass-formers , 4 294 Glauber s salt, 4 17... 

The applications of valence bond theory, the valence shell electron pair repulsion theory, to the bonding and bond angles of triatomic molecules... 

Valence Shell Electron Pair Repulsion Theory... 

Valence shell electron pair repulsion theory was applied to all the examples used in the chapter. 

The molecular structures adopted by simple carbonyl complexes are generally compatible with predictions based on valence shell electron pair repulsion theory. Three representative examples from the first transition series are shown in Fig. 15.2. 

VALENCE SHELL ELECTRON PAIR REPULSION THEORY 32... 

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... 

A recent electron diffraction study has shown that the structure is consistent with those predicted by the Valence Shell Electron Pair Repulsion Theory (137). 

Atoms are bound into molecules by shared pairs of electrons. Electrons dislike each other because like charges repel each other. Therefore, whether they are lone pairs of electrons or bonding pairs of electrons, electron pairs try to get as far apart in space as is geometrically possible. There is a fancy name that summarizes these simple ideas the VSEPR theory, which stands for Valence Shell Electron Pair Repulsion Theory. Even though the VSEPR theory is founded on fundamentally simple ideas, it is a tremendously powerful tool for predicting the shapes of molecules. 

The valence shell electron pair repulsion theory states the all electrons in a molecule mutually repel each other and achieve a geometry so that the bonding pairs and lone pairs of electrons are as far apart in space as possible. This theory allows one to predict the shape and geometry of a molecule. 

Various have been developed and theories put forward to find an answer to the questions like why the molecules acquire a particular shape and what decides the bond lengths, bond angles and bond strength of the bonds that hold atoms in a molecule. One of these is Valence Shell Electron Pair Repulsion Theory (VSEPR Theory). 

The shapes of molecules are determined by actual experiments, not by theoretical considerations. But we do not want to have to memorize the shape of each molecule. Instead, we would like to be able to look at a Lewis structure and predict the shape of the molecule. Several models enable us to do this. One of the easiest to use is valence shell electron pair repulsion theory, which is often referred to by its acronym VSEPR (pronounced vesper ). As the name implies, the theory states that pairs of electrons in the valence shell repel each other and try to stay as far apart as possible. You probably remember this theory from your general chemistry class. The parts of VSEPR theory that... 

If an attempt were made to apply the rules of valence shell electron pair repulsion theory to radicals, it would not be clear how to treat the single electron. Obviously, a single electron should not be as large as a pair of electrons, but it is expected to result in some repulsion. Therefore, it is difficult to predict whether a radical carbon should be sp2 hybridized with trigonal planar geometry (with the odd electron in a p orbital), sp3 hybridized with tetrahedral geometry (with the odd electron in an sp3 AO), or somewhere in between. Experimental evidence is also somewhat uncertain. Studies of the geometry of simple alkyl radicals indicate that either they are planar or, if they are pyramidal, inversion is very rapid. 

Valence shell electron pair repulsion theory... 

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