Bonds and molecular orbital theory

Valence bond and molecular orbital theory both incorporate the wave description of an atom s electrons into this picture of H2 but m somewhat different ways Both assume that electron waves behave like more familiar waves such as sound and light waves One important property of waves is called interference m physics Constructive interference occurs when two waves combine so as to reinforce each other (m phase) destructive interference occurs when they oppose each other (out of phase) (Figure 2 2) Recall from Section 1 1 that electron waves m atoms are characterized by their wave function which is the same as an orbital For an electron m the most stable state of a hydrogen atom for example this state is defined by the Is wave function and is often called the Is orbital The valence bond model bases the connection between two atoms on the overlap between half filled orbifals of fhe fwo afoms The molecular orbital model assembles a sef of molecular orbifals by combining fhe afomic orbifals of all of fhe atoms m fhe molecule... 

Two theories of bonding valence bond and molecular orbital theory,... 

For a molecule as simple as Fl2, it is hard to see much difference between the valence bond and molecular orbital methods. The most important differences appear- in molecules with more than two atoms. In those cases, the valence bond method continues to view a molecule as a collection of bonds between connected atoms. The molecular- orbital method, however, leads to a picture in which the sane electron can be associated with many, or even all, of the atoms in a molecule. We ll have more to say about the similarities and differences in valence bond and molecular- orbital theory as we continue to develop their principles, beginning with the simplest alkanes methane, ethane, and propane. 

The conclusion from this short discussion is that both valence bond and molecular orbital theories can describe the bonding of a system and... 

For a molecule as simple as H2, valence bond and molecular orbital theory produce very similar pictures. The next two sections describe these two approaches. 

A major difference between the valence-bond theory of chemical bonding and molecular orbital theory is that the former assumes, like the Lewis approach, that the electrons in a bond are localized between the two... 

Comparison of Qualitative Valence Bond and Molecular Orbital Theories... 

Although it is easy to demonstrate that benzene and other "aromatic" systems are stabilized, it is not as easy to determine the exact origin of the stabilization. Both valence bond and molecular orbital theories can provide a formalism for "explaining" the stabilization, and the latter can quantitatively account for the energy of benzene and its low reactivity. However, they do not provide a physical model for the stabilization. The latter must come from a consideration of the electron density distribution, for that alone determines the energy of a molecule. 

Valence bond and molecular orbital theories are alternative descriptions of chemical bonding. They have strengths and weaknesses, so they are complementary. Valence bond... 

The valence bond and molecular orbital theories differ in how they use the orbitals of two hydrogen atoms to describe the orbital that contains the electron pair in H2. Both theories assume that electron waves behave much like more familiar waves, such as sound and light waves. One property of waves that is important here is called interference in physics. Constructive interference occurs when two waves combine so as to reinforce each other ( in phase ) destructive interference occurs when they oppose each other ( out of phase ) (Figure 1.15). In the valence bond model constructive interference between two electron waves is seen as the basis for the shared electron-pair bond. In the molecular orbital model, the wave functions of molecules are derived by combining wave functions of atoms. 

Both modem theories of bonding, valence bond and molecular orbital theory, are based on the wave nature of an electron. Constructive interference between the electron wave of one atom and that of another gives a region between the two atoms in which the probabihty of sharing an electron is high—a bond. 

The more recent treatment of the covalent bond, based on the application of the principles of wave mechanics, has developed in two distinct forms, usually termed the valence-bond and molecular-orbital theories, respectively. Although ultimately there is no inconsistency between these two theories, they do in fact approach the problem of chemical binding from different points of view, and we shall generally find that for our purposes the valence-bond treatment is the more suitable. This theory starts from concepts already familiar to the chemist and its conclusions can usually be expressed verbally in qualitative terms the molecular-orbital theory, on the other hand, is more mathematical in its approach and lends itself less readily to such an interpretation. We shall, therefore, first discuss the valency-bond theory, and refer only briefly to the molecular-orbital treatment later in the chapter. 

The molecular orbital theory is more satisfying esthetically, perhaps, but its lack of emphasis on a localized chemical bond has led many chemists to prefer the valence bond method, which gives them a better pictorial grasp of the situation. The above distinction between the two methods is a primitive one. If all the refinements in the present day valence bond and molecular orbital theories are included, any distinction between them is probably more imagined than real. 

We submit that the Coulson-Fischer ansatz affords a third way in quantum chemistry that is distinct from the traditional valence bond and molecular orbital theories. In the next two sections, we very briefly survey the current state of the art in valence bond theory and in the multireference correlation problem based on the molecular orbital theory, before considering the Coulson-Fischer theory in more detail in Section 6. 

There is a recent beautiful discussion on the pluses and minuses of both valence-bond and molecular-orbital theory for the calculation and understanding of chemistry, and it is really worth reading [127]. 

This chapter walks you through the evolution of covalent bond theory, starting with the Lewis dot structures you likely covered in General Chemistry and then advancing to valence bond and molecular orbital theories that stem from quantum mechanics calculations. As with most chemistry, this chapter is purely about following the bouncing balls (electrons). 

Figure 11.20 Two possible formulae for dimethylaniline VALENCE BOND AND MOLECULAR ORBITAL THEORIES...

Both the valence bond and molecular orbital theories continue to be used today, and both involve approximations. Neither approach can claim to be final, and the chemist uses the method most suited to the problem in hand. 

Table 2. Approximate and Rigorous Types of Valence Bond and Molecular Orbital Theories. Theories within a Row are Equivalent.

Valence bond and molecular orbital theories are alternative descriptions of chemical bonding. They have strengths and weaknesses, so they are complementary. Valence bond theory is descriptively attractive, and it lends itself well to visualization. Molecular orbital theory gives better descriptions of electron cloud distributions, bond energies, and magnetic properties, but its results are not as easy to visualize. 

A A Use valence bond and molecular orbital theory to describe the change in Cl — Cl bond length when CI2 loses an electron to form Cl2. Would the cation be diamagnetic or paramagnetic Explain. 

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