Ionic modeling

It is important to apply a first principles technique since an alternative ionic modelling approach based on the fom-Gordon formalism yields errors in the cell parameters as high as... 

J.E. Post andC.W. Burnham, Ionic modeling of mineral structures and energies in the electron gas... 

Arrhenius success in science must be credited not only to his brilliance as a scientist but also to his conviction in his views. His understanding of the electrical properties of aqueous solutions nus so far ahead of contemporary thought that it would have been ignored but for his confidence in the usefulness of his theory and his refusal to abandon it. It is fitting tribute that the ionic model of aqueous solutions has changed permanently the face of inorganic chemistry. 

The model for the liquid phase may be obtained by analogy with the solid phase. Equation (11) is the ionic model reaction for... 

The ionic model, the description of bonding in terms of ions, is particularly appropriate for describing binary compounds formed from a metallic element, especially an s-block metal, and a nonmetallic element. An ionic solid is an assembly of cations and anions stacked together in a regular array. In sodium chloride, sodium ions alternate with chloride ions, and large numbers of oppositely charged ions are lined up in all three dimensions (Fig. 2.1). Ionic solids are examples of crystalline... 

Consider the closely related ion [FeCHiO/e] ". The only difference is in the formal oxidation state of the metal ion. If an ionic model is assumed (9.6), the charge on the metal center is +2. A purely covalent model results in the placing of a formal quadruple negative charge upon the iron center (9.7). To satisfy the electroneutrality principle, and establish a near-zero charge on the metal, each oxygen atom is... 

The ionic model describes a number of metal halides, oxides, and sulfides, but it does not describe most other chemical substances adequately. Whereas substances such as CaO, NaCl, and M 2 behave like simple cations and anions held together by electrical attraction, substances such as CO, CI2, and HE do not. In a crystal of Mgp2, electrons have been transferred from magnesium atoms to fluorine atoms, but the stability of HE molecules arises from the sharing of electrons between hydrogen atoms and fluorine atoms. We describe electron sharing, which is central to molecular stability, in Chapters 9 and 10. 

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. 

There are also molecules that are exceptions to the octet rule because one of the atoms has fewer, rather than more than, eight electrons in its valence shell in the Lewis structure (Figure 1.19). These molecules are formed by the elements on the left-hand side of the periodic table that have only one, two, or three electrons in their valence shells and cannot therefore attain an octet by using each of their electrons to form a covalent bond. The molecules LiF, BeCl2, BF3, and AIC13 would be examples. However, as we have seen and as we will discuss in detail in Chapters 8 and 9, these molecules are predominately ionic. In terms of a fully ionic model, each atom has a completed shell, and the anions obey the octet rule. Only if they are regarded as covalent can they be considered to be exceptions to the octet rule. Covalent descriptions of the bonding in BF3 and related molecules have therefore... 

Figure 8.7 Diborane, BaH. (a) Contour map of pb in the plane of the terminal hydrogens, (b) Contour map of pb in the plane of the bridging hydrogens, (c) Calculated geometry, (d) Experimental geometry. (e) Interatomic H-H distances, (f) Ionic model, (g) Resonance structures, (h) Protonated doublebond model, (i) VSEPR domain model showing the two three-center, two-electron bridging domains, (j) Hybrid orbital model.

Gillespie, R.J., Bytheway, I., Robinson, E.A. (1998). Bond lengths and bond angles in oxo, hydroxo, and alkoxo molecules of Be, B, and C A close-packed nearly ionic model. 

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. 

The state of the superoxide ion has been summarized by Naceache et al. 22). It appears probable that an ionic model is most suitable for the adsorbed species since the hyperfine interaction with the adjacent cation is relatively small. Furthermore, the equivalent 170 hyperfine interaction suggests that the ion is adsorbed with its internuclear axis parallel to the plane of the surface and perpendicular to the axis of symmetry of the adsorption site. Hence, the covalent structures suggested by several investigators have not been verified by ESR data. 

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