The language of valence bonding

It might be helpful at this point to summarize the concepts that VB theory has introduced into chemistry and which still survive even though MO theory is the domincuit computationcd mode  

The names of bond types o and n bonds are formed by spin pairing of electrons on adjacent atoms. 

Promotion valence electrons may be promoted to empty orbitals if overall that results in a lowering of energy. 

Hybridization atomic orbitals may be hybridized to match the observed geometry of a molecule. 

Resonance the superposition of individual structures. Resonance distributes multiple-bond character over the molecule and lowers the overall energy. 

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Both the language of valence bond theory and of molecular orbital theory are used in discussing structural effects on reactivity and mechanism. Our intent is to illustrate both approaches to interpretation. A decade has passed since the publication of the Third Edition. That decade has seen significant developments in areas covered by the text. Perhaps most noteworthy has been the application of computational methods to a much wider range of problems of structure and mechanism. We have updated the description of computational methods and have included examples throughout the text of application of computational methods to specific reactions. 

The VB and MO theories are both procedures for constructing approximations to the wavefunctions of electrons, but they construct these approximations in different ways. The language of valence-bond theory, in which the focus is on bonds between pairs of atoms, pervades the whole of organic chemistry, where chemists speak of o- and TT-bonds between particular pairs of atoms, hybridization, and resonance. However, molecular orbital theory, in which the focus is on electrons that spread throughout the nuclear framework and bind the entire collection of atoms together, has been developed far more extensively than valence-bond... 

Both the language of valence bond theory and resonance and that of molecular orbital theory are used in the discussion of structural effects on reactivity. Our intention is to illustrate the use of both typesr of interpretation, with the goal of facilitating the student s ability to understand and apply both of these viewpoints of structure. Nearly all reaction types and concepts are illustrated by specific examples from the chemical literature. Such examples, of course, cannot provide breadth of coverage, and those that are cited have been selected merely to illustrate the mechanism or interpretation. Such illustrations are not meant to suggest any... 

Among the countless concepts that Linus Pauling introduced from Quantum Mechanics into chemistry[l,2], and that became standard principles of the trade, there is the idea of hybridization . In the framework of the valence-bond description of a system, it is useful to mix atomic orbitals of the same n-quantum number , or of similar spatial extent, to construct directed, asymmetric atomic contributions. Although hybrids are not needed in an LCAO-MO description of the system, they have so much become part of the language of both organic and inorganic chemistry, that people will go out of their way to arrive at descriptions that are compatible with them. 

Using the language of conventional valence-bond structures, we say that a conjugated diene is a resonance hybrid of I and If. The dotted line in II represents... 

Electron flow paths are written in the language of Lewis dot structures and curved arrows. Lewis dot structures are used to keep track of all electrons, and curved arrows are used to symbolize electron movement. You must be able to draw a proper Lewis structure complete with formal charges accurately and quickly. Your command of curved arrows must also be automatic. These two points cannot be overemphasized, since all explanations of reactions will be expressed in the language of Lewis structures and curved arrows. A Lewis structure contains the proper number of electrons, the correct distribution of those electrons over the atoms, and the correct formal charge. We will show all valence electrons lone pairs are shown as darkened dots and bonds by lines. 

Using the language of the Valence Shell Electron Pair Repulsion Theory30 (VSEPR), this last trend could correlate with decreased electron-pair repulsion between the S—X and S—H bonding electrons. The more polarized S—X is towards X, the less the repulsion... 

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