Central atom concepts valence bond theory

Overall, hybrid orbitals provide a convenient model for using valence-bond theory to describe covalent bonds in molecules that have an octet or less of electrons around the central atom and in which the molecular geometry conforms to the electron-domain geometry predicted by the VSEPR model. While the concept of hybrid orbitals has limited predictive value, when we know the electron-domain geometry, we can employ hybridization to describe the atomic orbitals used by the central atom in bonding. 

In subsequent independent papers, Pauling [4] and Slater [6] generalized the valence-bond treatment made for the H2 molecule to polyatomic systems as H2O, NH3, CH4 etc. .. where an atom of the first period (the second row) is linked to hydrogens by several two-electron bonds they described the valence orbitals coming from the central atom by appropriate s and p combinations known later as hybrid orbitals. At the same time Hund [7] and Mulliken [8] presented another quantum theory of valence, the molecular orbital method in LCAO form, using the spectroscopic concept of molecular configuration built from s, p, d. ..pure atomic orbitals. The actual status of the hybridization process was clarified by Van Vleck [9], who showed that the various approximations... 

To predict the shapes of a variety of molecules we will use a theory which postulates that the shapes of molecules depend on the total number of bonded, and nonbonded or lone electron pairs surrounding a central atom. The chief concept is that electrons tend to repel each other, and that the mutual repulsion of all the electron pairs results in the molecules shape. The name of the method is therefore the valence-shell electron-pair repulsion theory, or the VSEPR theory. 

The molecular orbital (MO) is the basic concept in contemporary quantum chemistry. " It is used to describe the electronic structure of molecular systems in almost all models, ranging from simple Hiickel theory to the most advanced multiconfigurational treatments. Only in valence bond (VB) theory is it not used. Here, polarized atomic orbitals are instead the basic feature. One might ask why MOs have become the key concept in molecular electronic structure theory. There are several reasons, but the most important is most likely the computational advantages of MO theory compared to the alternative VB approach. The first quantum mechanical calculation on a molecule was the Heitler-London study of H2 and this was the start of VB theory. It was found, however, that this approach led to complex structures of the wave funetion when applied to many-electron systems and the mainstream of quantum ehemistry was to take another route, based on the success of the central-field model for atoms introduced by by Hartree in 1928 and developed into what we today know as the Hartree-Foek (HF) method, by Fock, Slater, and co-workers (see Ref. 5 for a review of the HF method for atoms). It was found in these calculations of atomic orbitals that a surprisingly accurate description of the electronic structure could be achieved by assuming that the electrons move independently of each other in the mean field created by the electron cloud. Some correlation was introduced between electrons with... 

What most clearly distinguishes bond models from other models of chemical stmcture is the centrality of the concept of valence, that is, the number of electrons that an atom uses for bonding. In order to determine the valence, we must be able to count how many valence electrons each atom contains. This is not possible in electron density models such as the quantum theory of atoms in molecules (QTAIM), where the individuality of the electron is lost as soon as it enters the atom [5]. A different picture of the atom is needed, one that tracks the functions of the individual electrons, rather than their locations. 

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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