Valence bond theory, definition

Earlier in this chapter, you learned the definition of bond order in the valence bond theory. In molecular orbital theory, the bond order is defined as one-half the difference between the number of electrons in bonding orbitals and the number of electrons in antibonding orbitals. Mathematically, this can be expressed as... 

Aromaticity is one of a number of remarkably important but fuzzy concepts in chemistry that does not lend itself easily to widely applicable clear-cut definitions. One of the more recent attempts to summarise what chemists with dilferent backgrounds see in this term was made by Schleyer. Valence bond theory has been, since Kekule s times, a major source of qualitative and quantitative interpretations of aromaticity, and it is surprising that it has not been mentioned, even once, in Schleyer s editorial. Of course, this omission does not diminish the importance that VB methods have had and still have in providing easy-to-understand ideas about aromatic behaviour which are often markedly superior to those coming from other theoretical sources. In order to justify this claim from a current perspective, in this section we look at the recent VB studies of the aromaticity of a series of benzenelike rings, and of clamped benzenes. 

The first point to be made concerning acids and bases is that so-called acid-base theories are in reality definitions of what an acid or base is they are not theories in the sense of valence bond theory or molecular orbital theory. In a very real sense, we can make an acid be anything we wish the differences between the various acid-base concepts are not concerned with which is right but which is most convenient to use in a particular situation. All of the current definitions of acid-base behavior are compatible with each other. In fact, one of the objects in the following presentation of many different definitions is to emphasize their basic parallelism and hence to direct the students toward a cosmopolitan attitude toward acids and bases which will stand them in good stead in dealing with various chemical situations, whether they be in aqueous solutions of ions, organic reactions, nonaqueotis titrations, or other situations. 

Definition of Bonding Energies in Valence Bond Theory... 

Molecular geometry is the general shape of a molecule, as determined by the relative positions of the atomic nuclei. There is a simple model that allows you to predict molecular geometries, or shapes, from Lewis formulas. This valence-shell electron-pair model usually predicts the correct general shape of a molecule. It does not explain chemical bonding, however. For this you must look at a theory, such as valence bond theory, that is based on quantum mechanics. Valence bond theory provides further insight into why bonds form and, at the same time, reveals that bonds have definite directions in space. 

Coulson (1939) contributed still another paper in which the notion of fractional bond orders was introduced for the first time in the context of molecular orbital theory. Fractional bond orders formalized the notion entertained by many chemists for a long time that in many substances, of which benzene is the paradigmatic example, bonds appear to have an intermediate character between single and double bonds. Hiickel was the first to introduce the distinction between a- and w-electrons in benzene, and it was in the framework of valence bond theory that the first definition... 

How does each of the three major bonding theories (the Lewis model, valence bond theory, and molecular orbital theory) define a single chemical bond A double bond A triple bond How are these definitions similar How are they... 

From the earliest days of the electronic theory of valency attempts (see p. 959) were made to explain the structure and reactions of organic compounds in terms of the behaviour of the electrons in their molecules. Information about these can be obtained from absorption spectra. Another thing requiring explanation was the definite directions in space of valency bonds, which is required by the facts of stereochemistry and optical activity, e.g. that the four... 

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