Chemical bonding VSEPR theory

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. 

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. 

VSEPR theory indicates nothing about the nature of the chemical bond (localized Heitler-London versus delocalized Hund-Mulliken). It simply predicts the geometrical shape, specifically, the X-X, M-X and/or X-M-X bond angles in the molecule. 

Theory is a term that is very widely used by chemists. To take the area of chemical bonding as an example, chemists widely refer to molecular orbital (hereafter MO) theory, valence bond (VB) theory, hybridization theory, valence shell electron pair repulsion (VSEPR) theory, and ligand field theory. And even those probably do not exhaust the list. 

By use of VSEPR theory, in which the Lewis structures helped us determining the number of surrounding electron groups, we are now able to predict actual structures of molecules and composite ions. However from the VSEPR theory we know nothing about the chemical bond itself Where are the bond electrons actually placed Or more specific In which types of orbitals are the bond electrons placed The answer to this can be found in the orbital hybridization theory which is the topic in the next section. 

However the orientations of the atomic orbitals in space do not fit the directions predicted by the VSEPR theory according to Table 2- 1 on page 70. For this reason other orbitals than the atomic orbitals must be present in the molecules and composite ions in order to give the right bond directions according to the VSEPR theory. These orbitals are a type of molecular orbitals (also mentioned in section 2.2.2 Molecular orbital theory) which are called hybrid orbitals. These hybrid orbitals thus host the valence electrons which constitutes the chemical bond between the atoms. 

The first ehapter emphasizes the current method employed in mathematical calculations, viz., factor-label analysis. In the section on chemical bonding, although molecular orbitals are discussed, VSEPR theory (valenee-shell electron-pair repulsion theory) is emphasized in characterizing three-dimensional moleeular structure. The discussion of nuclear processes includes material on modem spectroscopic methods of noninvasive anatomical visualization. 

The simplicity of the VSEPR model is one of its primary strengths. In addition, the model provides a continuity in the development of the qualitative ideas about the nature of the chemical bond and its correlation with molecular structure. Abegg s octet rule (see, e.g.. Ref. [3-71]) and Lewis s theory of the shared electron pair [3-72] may be considered as direct forerunners of the model. 

It is important to recognize at this point that VBT Is only a model. As such, it is useful only to the extent that it helps us visualize and better understand the nature of the chemical bonding in molecules. Given the simplicity of Lewis structures, the intricacies of the VSEPR model, and the relatively straightforward concept of hybridization, it Is remarkable how far the valence bond model can be pushed in the first place. In the next section, MOT will be introduced as an alternative and complementary model to VBT. In many ways, MOT will prove itself to be the more powerful of the two theories in that it can explain most of the shortcomings of the... 

Finally, in addition to simply representing a pair of shared electrons, a chemical bond has structural implications as well. Because electrons are negatively charged, when there are several distinct bonds, they will tend to be physically separated from each other. This idea is the basis for a method to predict the geometry of molecules called the Valence Shell Electron Pair Repulsion (VSEPR) theory. Using this theory, the general shape of molecules and ions can be predicted. 

The VSEPR model, based largely on Lewis structures, provides a relatively simple and straightforward method for predicting the geometry of molecules. But as we noted earlier, the Lewis theory of chemical bonding does not clearly explain why chemical bonds exist. Relating the formation of a covalent bond to the pairing of electrons was... 

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