Chemical bonding ionic model

The resulting substance is sometimes said to contain an ionic bond. Indeed, the properties of a number of compounds can be adequately explained using the ionic model. But does this mean that there are really two kinds of chemical bonds, ionic and covalent ... 

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

Ionic and covalent bonding are two extreme models of the chemical bond. Most actual bonds lie somewhere between purely ionic and purely covalent. When we describe bonds between nonmetals, covalent bonding is a good model. When a metal and nonmetal are present in a simple compound, ionic bonding is a good model. However, the bonds in many compounds seem to have properties between the two extreme models of bonding. Can we describe these bonds more accurately by improving the two basic models ... 

The main handicap of MD is the knowledge of the function [/( ). There are some systems where reliable approximations to the true (7( r, ) are available. This is, for example, the case of ionic oxides. (7( rJ) is in such a case made of coulombic (pairwise) interactions and short-range terms. A second example is a closed-shell molecular system. In this case the interaction potentials are separated into intraatomic and interatomic parts. A third type of physical system for which suitable approaches to [/( r, ) exist are the transition metals and their alloys. To this class of models belong the glue model and the embedded atom method. Systems where chemical bonds of molecules are broken or created are much more difficult to describe, since the only way to get a proper description of a reaction all the way between reactant and products would be to solve the quantum-mechanical problem at each step of the reaction. 

On the basis of the shell model, two apparently different models of the chemical bond were proposed, the ionic model and the covalent model. 

Formal charge and oxidation number are two ways of defining atomic charge that are based on the two limiting models of the chemical bond, the covalent model and the ionic model, respectively. We expect the true charges on atoms forming polar bonds to be between these two extremes. 

Robinson, E.A., Johnson, S.A., Tang, T.-H., Gillespie, R.J. (1997). Reinterpretation of the lengths of bonds to fluorine in terms of an almost ionic model. Inorganic Chemistry, 36, 3022-3030. Schinder, H.L. Becke, A.D. (2000). Chemical contents of the kinetic energy density. Journal of Molecular Structure (THEOCHEM), 527, 51-61. 

However, It has been found that in many cases, simple models of the properties of atomic aggregates (monocrystals, poly crystals, and glasses) can account quantitatively for hardnesses. These models need not contain disposable parameters, but they must be tailored to take into account particular types of chemical bonding. That is, metals differ from covalent crystals which differ from ionic crystals which differ from molecular crystals, including polymers. Elaborate numerical computations are not necessary. 

These speculations about the ionic, polar, or electronic nature of chemical bonding, which arose largely from solution theory, resulted mostly in static models of the chemical bond or atom structure. In contrast is another tradition, which is more closely identified with ether theory and electrodynamics. This tradition, too, may be associated with Helmholtz, especially by way of his contributions to nineteenth-century theories of a "vortex atom" that would explain chemical affinities as well as the origin of electromagnetism, radiation, and spectral lines. 

Theoretical aspects of the bond valence model have been discussed by Jansen and Block (1991), Jansen et al. (1992), Burdett and Hawthorne (1993), and Urusov (1995). Recently Preiser et al. (1999) have shown that the rules of the bond valence model can be derived theoretically using the same assumptions as those made for the ionic model. The Coulomb field of an ionic crystal naturally partitions itself into localized chemical bonds whose valence is equal to the flux linking the cation to the anion (Chapter 2). The bond valence model is thus an alternative representation of the ionic model, one based on the electrostatic field rather than energy. The two descriptions are thus equivalent and complementary but, as shown in Section 2.3 and discussed further in Section 14.1.1, both apply equally well to all types of acid-base bonds, covalent as well as ionic. 

This book is divided into four parts. Part I provides a theoretical derivation of the bond valence model. The concept of a localized ionic bond appears naturally in this development which can be used to derive many of its properties. The remaining properties, those dependent on quantum mechanics, are, as in the traditional ionic model, fitted empirically. Part II describes how the model provides a natural approach to understanding inorganic chemistry while Part 111 shows how the limitations of three-dimensional space lead to new and unexpected properties appearing in the inorganic chemistry of solids. Finally, Part IV explores applications of the model in disciplines as different as condensed matter physics and biology. The final chapter examines the relationship between the bond valence model and other models of chemical bonding. 

The reader s attention is drawn to the discussion in Sections 14.3 and 14.4 which shows that all chemical bond models are equivalent because they all reduce to this same topological description. The derivation here is based on the ionic model because it is the simplest and most convincing. 

As mentioned in Section 2.4, in the ionic model the chemical bond is an electrical capacitor. It is therefore possible to replace the bond network by an equivalent electric circuit consisting of links which contain capacitors as shown in Fig. 2.6. The appropriate Kirchhoff equations for this electrical network are eqns (2.7) and (2.11). It is thus possible in principle to determine the bond fluxes for a bond network in exactly the same way as one solves for the charges on the capacitors of an electrical network. While solving these equations is simple in principle providing the capacitances are known, the calculation itself can be... 

This chapter shows that the ionic model can not only be presented in terms of chemical bonds characterized by their electrostatic flux, but also that the improbable assumptions of the model are satisfied by the wide range of compounds that conform to the following two conditions ... 

Although the empirical eqns (3.3) and (3.4) can be justified by their similarity with eqns (2.7) and (2.11) which have been derived using the ionic model, they are not restricted to ionic bonds. The formation of a chemical compound results in the pairing of the unpaired valence electrons drawn from the two bonded... 

Valence bond (VB) theories or empirical valence bond (EVB) methods have been developed in order to solve this problem with bond potential functions that (i) allow the change of the valence bond network over time and (ii) are simple enough to be used efficiently in an otherwise classical MD simulation code. In an EVB scheme, the chemical bond in a dissociating molecule is described as the superposition of two states a less-polar bonded state and an ionic dissociated state. One of the descriptions is given by Walbran and Kornyshev in modeling of the water dissociation process.4,5 As... 

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