Valence-shell electron-pair repulsion shape

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)... 

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 Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

Example the n = 2 shell of Period 2 atoms, valence-shell electron-pair repulsion model (VSEPR model) A model for predicting the shapes of molecules, using the fact that electron pairs repel one another. 

The most stable shape for any molecule maximizes electron-nuclear attractive interactions while minimizing nuclear-nuclear and electron-electron repulsions. The distribution of electron density in each chemical bond is the result of attractions between the electrons and the nuclei. The distribution of chemical bonds relative to one another, on the other hand, is dictated by electrical repulsion between electrons in different bonds. The spatial arrangement of bonds must minimize electron-electron repulsion. This is accomplished by keeping chemical bonds as far apart as possible. The principle of minimizing electron-electron repulsion is called valence shell electron pair repulsion, usually abbreviated VSEPR. 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

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)... 

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 stands for Valence Shell Electron Pair Repulsion and these electron pair repulsions are responsible for the shapes of molecules and polyatomic ions, such as NH+. 

We ll start with the hard part VSEPR stands for valence shell electron pair repulsion. Okay, now it gets easier. VSEPR is simply a model that helps predict and explain why molecules have the shapes they do. Moleculcir shapes help determine how molecules interact with each other. For example, molecules that stack nicely on one another are more likely to form solids. And two molecules that can fit together so their reactive bits lie closer together in space are more likely to react with one another. 

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 ... 

Molecular shape can often be predicted by the valence-shell electron-pair repulsion (VSEPR) model, which treats... 

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... 

For xenon fluorides and oxides, for example, the same models can be apphed as for interhalogen and halogen oxy species. Furthermore, the very successful valence shell electron pair repulsion (VSEPR) rules for molecule and ion shapes are as effective for noble gas compounds and their relatives as for classical octet compounds. 

The P-donor ligands we consider in this Part I are phosphoms(III) compounds. We avoid the classification difficulties of phosphine, PH3, as based on P-oxidation state —HI by referring throughout to tervalent phosphoms. Low-coordination number phosphorus species, such as RP, RCP, R2C=PR, and P , are presented in Part II. The P-donor ligands considered in Part I are covered by the generalized formula PR3 for a P-donor ligand. The PR3 ligands have a pyramidal shape due to their sterically active lone pair of electrons. In terms of a Valence Shell Electron Pair Repulsion Model model, the lone pair occupies the vacant tetrahedral site of the phosphoms center. 

VSEPR (Section 1.6B) Valence Shell Electron Pair Repulsion theory. A theory that determines the three-dimensional shape of a molecule by the number of groups surrounding a central atom. The most stable arrangement keeps the groups as far away from each other as possible. 

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. 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

Obviously, the formulas CO2 and SO2 do not provide any information about the shapes of these molecules. However, there is a model that can be used to predict the shape of a molecule. This model is based on the valence shell electron pair repulsion (VSEPR) theory. Using this model, you can predict the shape of a molecule by examining the Lewis structure of the molecule. 

Once a Lewis structure is drawn, you can determine the molecular geometry, or shape, of the molecule. The model used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion model, or VSEPR model. This model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. 

The Shapes of Molecules Valence Shell Electron-Pair Repulsion Theory... 

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