The Structure and Reactivity of Molecules

Valence Shell Electron Pair Repulsion Theory  

In this chapter a few simple rules for predicting molecular structures will be invesli-galed. We shall examine first the valence shell electron pah repulsion (VSEPR) model, and then a purely molecular orbital treatment. 

We begin by considering the simplest molecules—those in which the electrons on the central atom are all involved in bonds. It should be kept in mind that each molecule is a unique structure resulting from the interplay of several energy factors and that the following rules can only be a crude attempt to average the various forces. 

from the electronic configuration of the elements, determine a reasonable Lewis structure. For example, in the carbon dioxide molecule, there will be a total of 16 valence electrons to distribute among three atoms  

Note that a Lewis structure says nothing about the bond angles in the molecule since both (a) and (b) meet all the criteria for a valid Lewis structure. 

---

Most chemists still tend to think about the structure and reactivity of atomic and molecular species in qualitative terms that are related to electron pairs and to unpaired electrons. Concepts utilizing these terms such as, for example, the Lewis theory of valence, have had and still have a considerable impact on many areas of chemistry. They are particularly useful when it is necessary to highlight the qualitative similarities between the structure and reactivity of molecules containing identical functional groups, or within a homologous series. Many organic chemistry textbooks continue to use full and half-arrows to indicate the supposed movement of electron pairs or single electrons in the description of reaction mechanisms. Such concepts are closely related to classical valence-bond (VB) theory which, however, is unable to compete with advanced molecular orbital (MO) approaches in the accurate calculation of the quantitative features of the potential surface associated with a chemical reaction. 

---