The Nature of Chemical Bonds Valence Bond Theory

5 I The Nature of Chemical Bonds Valence Bond Theory 

How does electron sharing lead to bonding between atoms Two models have been developed to describe covalent bonding valence bond theory and molecular orbital theory. Each model has its strengths and weaknesses, and chemists tend 

Released when bond forms Absorbed when bond breaks 

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THE NATURE OF CHEMICAL BONDS VALENCE BOND THEORY... 

Atomic Structure The Nucleus Atomic Structure Orbitals 4 Atomic Structure Electron Configurations 6 Development of Chemical Bonding Theory 7 The Nature of Chemical Bonds Valence Bond Theory sp Hybrid Orbitals and the Structure of Methane 12 sp Hybrid Orbitals and the Structure of Ethane 13 sp2 Hybrid Orbitals and the Structure of Ethylene 14 sp Hybrid Orbitals and the Structure of Acetylene 17 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur 18 The Nature of Chemical Bonds Molecular Orbital Theory 20 Drawing Chemical Structures 21 Summary 24... 

With the development of quantum mechanics, however, this knowledge is providing much of the experimental basis for explanations of the nature of chemical bonds. The first steps, taken by applying quantum mechanical ideas to the earlier electron theories of valence reinforced by the quantitative measurements of spectroscopy, identified many bonds in terms of the electrons that could form them. Thus, hybridization of one s and three p electrons was shown to account for the tetrahedrally distributed four equal... 

Theoretical investigations into the nature of chemical bonding were centred around valence bond (VB) theory and molecular orbital (MO) theory. 

Spectroscopy and the electron theory of valence provided valuable support for one another. Together, they took our understanding of the nature of chemical elements to a new level, where chemical behavior and chemical structure could both be interpreted in terms of the number and disposition of electrons in the atoms of any given element. At least, the simplified model of atomic orbitals brilliantly developed by Linus Pauling enabled him to explain and predict a great deal of chemistry, in terms of bonds and structures. 

Valence-Bond (VB) and Molecular-Orbital (MO) theories both were clearly formulated by the end of the first decade of quantum mechanics. Of course VB theory is connected to early conceptual roots in chemistry, as emphasized by Rumer [1] and more particularly by Pauling, in a review [2] and then in his masterwork [3] The Nature cfiJte Chemical Bond. Thence for some jjeriod of time VB theory seems in the chemical community to have been viewed quite favorably. 

The first mention of the methylene molecule in the traditional scientific literature seems to be by Mulliken [5] in a 1932 Physical Review article titled Electronic Structures of Polyatomic Molecules and Valence, n. Quantum Theory of the Chemical Bond. Mulliken was interested in the nature of double bonds and in particular the double bond in ethylene, which he analyzed in terms of the constituent CH2 fragments. In the course of this analysis he proposed that the ground state of CH2 was of Ai symmetry with an angle of about 110° and that there was a low-lying Bi state. This was a remarkable illustration of Mulliken s legendary insight. It is humbling to note that quantum mechanics as we know it was only 6 years old. 

Quantum Mechanics offers the most comprehensive and most successful explanation of many chemical phenomena such as the nature of valency and bonding as well as chemical reactivity. It has also provided a fundamental explanation of the periodic system of the elements which summarizes a vast amount of empirical chemical knowledge. Quantum Mechanics has become increasingly important in the education of chemistry students. The general principles provided by the theory mean that students can now spend less time memorizing chemical facts and more time in actually thinking about chemistry. 

Some 50 years have now passed since the publication of a series of papers bearing the title The Nature of the Chemical Bond. 1 7 These papers have provided chemists, physicists, biologists, and mineralogists with the conceptual framework, based on simple valence bond theory and the theory of hybrid bond orbitals, required to investigate a myriad of problems involving the nature of the bonding exhibited in molecules and solids. The ideas contained in these papers were subsequently elaborated on in The Nature of the Chemical Bond which is probably the most often-cited book in the scientific literature.9... 

The NRT resonance weights, bond orders, and valencies are generally comparable to those of the older Pauling-Wheland theory (particularly for species of low ionicity) and can be used to rationalize chemical phenomena in a similar fashion. Pauling s classic, The Nature of the Chemical Bond, brilliantly illustrates such reasoning. 

The classic HLSP-PP-VB (Heitler-London-Slater-Pauling perfect-pairing valence-bond) formalism and its chemical applications are described by L. Pauling, The Nature of the Chemical Bond. 3rd edn. (Ithaca, NY, Cornell University Press, 1960 G. W. Wheland, The Theory of Resonance (New York, John Wiley, 1944) and H. Eyring, J. Walter, and G. E. Kimball, Quantum Chemistry (New York, John Wiley, 1944). 

In the nineteenth century the valence bond was represented by a line drawn between the symbols of two chemical elements, which expressed in a concise way many chemical facts, but which had only qualitative significance with regard to molecular structure. The nature of the bond was completely unknown. After the discovery of the electron numerous attempts were made to develop an electronic theory of the chemical bond. These culminated in the work of Lewis, who in... 

Because the n-networks of benzenoid hydrocarbons are the classical prototypical example of delocalized bonding, they provide a crucial test for chemical-bonding theories. Here there is revealed a systematic organization for valence-bond views to describe such n-bonding. With an initiation near the ab initio realm a sequence of semiempirical valence-bond models is identified and characterized. The refinement from one model to the next proceeds via either a (perturbative) restriction to a smaller model space or orthogonalization of a suitable natural basis for the model space. The known properties of the models are indicated, and possible methods of solution are mentioned. A great diversity of work is outlined, related, systematized and extended. New research is suggested. 

The contributions of Erich Hiickel to the development of molecular orbital theory have already been mentioned in the subsection on Germany (Section 5.4.1) the development of semi-empirical quantum mechanical treatments in organic chemistry by M. J. S. Dewar has been discussed in Section 5.5. In the early development of the application of quantum mechanics to chemistry, Linus Pauling (1901-1994)359 was pre-eminent. He was associated with CalTech for most of his career. His work before World War II generated two influential books the Introduction to Quantum Mechanics (with E. Bright Wilson, 1935)360 and The Nature of the Chemical Bond (1939).361 He favoured the valence-bond treatment and the theory of resonance. 

Despite the quantitative victory of molecular orbital (MO) theory, much of our qualitative understanding of electronic structure is still couched in terms of local bonds and lone pairs, that are key conceptual elements of the valence bond (VB) picture. VB theory is essentially the quantum chemical formulation of the Lewis concept of the chemical bond [1,2]. Thus, a chemical bond involves spin-pairing of electrons which occupy valence atomic orbitals or hybrids of adjacent atoms that are bonded in the Lewis structure. In this manner, each term of a VB wave function corresponds to a specific chemical structure, and the isomorphism of the theoretical elements with the chemical elements creates an intimate relationship between the abstract theory and the nature of the... 

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