VSEPR theory shapes

The shapes of covalent compounds are determined by the tendency for bonding pairs to be as far apart as possible whilst lone pairs have a greater effect than bonding pairs (VSEPR theory). 

STRATEGY The existence of residual entropy at T = 0 suggests that the molecules are disordered. From the shape of the molecule (which can be obtained by using VSEPR theory), we need to determine how many orientations, W, it is likely to be able to adopt in a crystal then we can use the Boltzmann formula to see whether that number of orientations leads to the observed value of S. 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

Table 1.4 Shapes of Molecules and Ions from VSEPR Theory...

Use the VSEPR theory to work out the shapes of the following molecules or ions. 

The result here is quite satisfactory because XeF4 does in fact exhibit square planar geometry. It is worth noting, however, that a square planar shape for XeF4 is also predicted by VSEPR theory. Despite the fact that the molecular orbital method has made some inroads as of late, VSEPR is still the best approach available for rationalizing the molecular geometries of noble gas compounds. 

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... 

You can use soap bubbles to simulate the molecular shapes that are predicted by VSEPR theory. The soap bubbles represent the electron clouds surrounding the central atom in a molecule. 

Determine the shape of SiFe " using VSEPR theory. 

Use VSEPR theory to predict the molecular shape for each of the following ... 

Draw Lewis structures for the following molecules and ions, and use VSEPR theory to predict the molecular shape. Indicate the examples in which the central atom has an expanded octet. 

Use VSEPR theory to predict the shape of each of the following molecules. From the molecular shape and the polarity of the bonds, determine whether or not the molecule is polar. 

Q iai f For the compound, GeH4, use VSEPR theory to determine its molecular shape and indicate whether or not this molecule is polar. 

CD If possible for your material, draw a Lewis structure of the molecule or molecules on which your material is based. Predict the molecular shape using VSEPR theory. 

Use VSEPR theory to predict the molecular shape of CH2CI2. Draw a sketch to indicate the polarity of the bonds around the central atom to verify that this is a polar molecule. 

Use VSEPR theory to determine the shape of each molecule. 

Shaping up molecules with VSEPR theory and hybridization... 

VSEPR theory predicts several shapes that appear over and over in real-life molecules. You can see them in Figure 5-11. 

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.)... 

In order for VSEPR theory to make sense, it must be combined with another idea hybridization. Hybridization refers to the mixing of atomic orbitals into new, hybrid orbitals of equal energy. Electron pairs occupy equivalent hybrid orbitals. It s important to realize that the hybrid orbitals cire all equivalent, because that helps you understand the shapes that emerge from the electron pairs trying to distance themselves from one another. If electrons in a pure p orbital are trying to distance themselves from electrons in another p orbital and from electrons in an s orbital, the resulting shape may not be symmetrical, because s orbitals cire different from p orbitals. But if all these electrons occupy identical hybrid orbitals (each orbital is a little bit s and a little bit p), then the resulting shape is more likely to be symmetrical. 

Real molecules have all sorts of symmetrical shapes that just don t make sense if electrons truly occupy only pure orbitals (like s and p). The mixing of pure orbitals into hybrids allows chemists to explain the symmetrical shapes of real molecules with VSEPR theory. This kind of mixing must in some sense actually occur, as the case of methane, CH, makes clear. 

The single 2s orbital combines with the three 2p orbitals to create four identical sp hybrid orbitals. The fact that each sp orbital is identical is important because VSEPR theory can now explain the symmetrical shape of methane the tetrahedron. 

The extension of molecular orbital theory to triatomic molecules is a major part of this chapter. It gives a very satisfactory description of the shapes and the bonding of molecules in general, and is consistent with observations of photoelectron and electronic absorption spectra. It is not possible for the VSEPR theory to explain these latter observations. 

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. 

Two major theories of the covalent bond are described in this book the main features of valence bond theory are treated in terms of the VSEPR theory of molecular shapes, and MO theory which is based on the symmetry properties of the contributing atomic orbitals. The latter theory is applied qualitatively with MO diagrams being constructed and used to interpret bond orders and bond angles. The problems associated with bond angles are best treated by using the highest symmetry possible for a molecule of a particular stoichiometry. 

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-... 

Figure 6.17 A summary of the predictions of molecular shape by the VSEPR theory...

Figure 6.18 Examples of VSEPR theory predictions of molecular shapes in a series of hypothetical dehydration reactions. The element E is covalently saturated...

Hydrogen fluoride reacts with antimony pentasulfide to give the ions H,F+ and SbF6. Predict the shapes of the two ions using VSEPR theory and then use the MO approach to describe their bonding. 

Throughout the book, theoretical concepts and experimental evidence are integrated An introductory chapter summarizes the principles on which the Periodic Table is established and describes the periodicity of various atomic properties which are relevant to chemical bonding. Symmetry and group theory are introduced to serve as the basis of all molecular orbital treatments of molecules. This basis is then applied to a variety of covalent molecules with discussions of bond lengths and angles and hence molecular shapes. Extensive comparisons of valence bond theory and VSEPR theory with molecular orbital theory are included Metallic bonding is related to electrical conduction and semi-conduction. 

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