Atomic orbitals modem view

Modem views of atomic structure are, as we have seen, based largely on the applications of wave mechanics to atomic systems. Modem views of molecular structure are based on applying wave mechanics to molecules such studies provide answers as to how and why atoms combine. The Schrodinger equation can be written to describe the behaviour of electrons in molecules, but it can be solved only approximately. Two methods of doing this are the valence bond approach, developed by Heitler and Pauling, and the molecular orbital approach associated with Hxmd and MuUiken ... 

The foregoing discussion of valence is. of course, a simplified one. From ihe development of the quantum theory and its application to the structure of the atom, there has ensued a quantum theory of valence and of the structure of the molecule, discussed in this hook under Molecule. Topics thal are basically important to modem views of molecular structure include, in addition to those already indicated the Schroedinger wave equation the molecular orbital method (introduced in the article on Molecule) as well as directed valence bonds bond energies, hybrid orbitals, the effect of Van der Waals forces and electron-dcticiem molecules. Some of these subjects are clearly beyond the space available in this book and its scope of treatment. Even more so is their use in interpretation of molecular structure. [However, sec Crystal Field Theory and Ligand.)... 

It should by now be clear from the preceding sections that the generally accepted view of atomic orbitals is that they are strictly non-referring in that they do not pick out any entity which may be said to physically exist in the same sense that a planetary orbit exists. This does not, however, diminish their usefulness in a multitude of computational schemes employed in quantum chemistry or their applicability to the classification of spectroscopic transitions and to a discussion of a plethora of chemical and physical phenomena. In addition, much of the success of modem quantum chemistry, including the award of the 1998 Nobel Prize for chemistry, is directly attributable to the use of calculations based on atomic and molecular orbitals. 

I claim that atomic orbitals provide an interesting prima facie case against this view since they began life as well defined referential terms in Bohr s, and even older theories of atomic structure, only to lose their ability to refer with the advent of quantum mechanics. It does not matter whose orbitals are selected from the modem palette of choices since none of them refer. The realist cannot therefore claim that reference has been preserved in the same way that she might in the case of electron theories. 

The concept of a chemical bond as a localized interaction between two neighboring atoms has been a central part of chemistry for the past century and a half, yet our current description of chemical bonds is still empirical it is a collage of ill-defined and largely incompatible models that are based on assumptions that do not always correspond to physical reality. The ionic and covalent models are mutually incompatible, and both the Lewis and orbital models have serious flaws [3, 4]. They do not conform to modem views of atomic stmcture, and consequently their predictions sometimes fail. While the bond valence theory belongs to this tradition of localized bond models, it is derived from a realistic, though simplified picture of the atom, one that is compatible with more sophisticated atomic descriptions. It can be used to derive powerful and quantitative theorems about chemical stracture. The mles of both the traditional ionic and covalent models can be derived as two special cases of this model (Sects. 5 and 7.2). 

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