Simple Bonding Theory

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of our discussion of chemical bonding uses the molecular orbital approach, less rigorous methods that provide approximate pictures of the shapes and polarities of molecules are also useful. This chapter provides an overview of Lewis dot structures, valence shell electron-pair repulsion (VSEPR), and related topics. Molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters the ideas of this chapter provide a starting point for that more modem treatment. 

Simple molecules such as water follow the octet rule, in which eight electrons surround the central atom. Each hydrogen atom shares two electrons with the oxygen, forming the familiar structure with two bonds the O atom accommodates two bonding pairs and two lone pairs  

Shared electrons are considered to contribute to the electronic requirements of both atoms involved thus, the electron pairs shared by H and O in the water molecule are counted toward both the 8-electron requirement of oxygen and the 2-electron requirement of hydrogen. 

The Lewis model defines double bonds as containing four electrons and triple bonds as containing six electrons  

The species C03 and N03 have the same number of electrons (i.e., they are isoelectronic) and use the same orbitals for bonding. Their Lewis diagrams are identical except for the identity and formal charge (Section 3.1.3) of the central atom. 

A more detailed approach to electron-dot diagrams is presented in Appendix D. 

Some bonds are double bonds, containing four electrons, or triple bonds, containing six electrons  

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The study of 8-N compounds is one of the most active areas of current inorganic research many novel cyclic and acyclic compounds are being prepared which have unusual stmctures and which pose considerable problems in terms of simple bonding theory. The discoveiy in 1975 that the polymer (8N)jc is a metal whose conductivity increases with decrease in... 

Simple bonding theory and the carbon monoxide molecule ... 

We can divide the NAOs into the natural minimal basis (NMB), consisting of only the formal core and valence NAOs of simple bonding theory, and the natural Rydberg basis (NRB), consisting of everything else. With this partitioning, Eq. (3.24a) is expressed as... 

Section 8.5). There is no simple bonding theory to account for the existence of such compounds and their stoichiometries. You will often see reference made to materials such as ZrCl, and you must suspect a complex metallic structure rather than a simple ionic array in such cases. 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

Chapters Simple Bonding Theory TABLE 3.6 Bond Angles and Lengths... 

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