Models of Chemical Bonding

Binding Atoms Together The properties of substances, such as the low conductivity and high melting point of sodium chloride crystals, depend on the properties of their atoms and how they bind together, as you ll see in this chapter. 

Most covalent compounds are molecular substances with low melting and boiling points, because these physical changes disrupt the weak attractions befween the molecules wNIe leaving the strong covalent bonds within the molecules intact. 

During a reaction, energy is absorbed to break certain bonds in the reactant molecules and is released to form other bonds that create the product molecules the heat of reaction is the difference between the energy absorbed and the energy releasea 

Each atom in a covalent bond attracts the shared electron pair according to its electronegativity (BNJ. A covalent bond is polar if the two atoms have different EN values The feme character of a bond— from highly ionic to nonpolar covalent— varies with the difference in EN values of the atoms 

1 Atomic Properties and Chemical Bonds 9.3 The Covalent Bonding Model 9.5 Between the Extremes Electronegativity 

The familiar Lewis structure is the simplest bonding model in common use in organic chemistry. It is based on the idea that, at the simplest level, the ionic bonding force arises from the electrostatic attraction between ions of opposite charge, and the covalent bonding force arises from sharing of electron pairs between atoms. 

The starting point for the Lewis structure is a notation for an atom and its valence electrons. The element symbol represents the core, that is, the nucleus and all the inner-shell electrons. The core carries a number of positive charges equal to the number of valence electrons. This positive charge is called the corechanie. Valence electrons are shown explicitly. For elements in the third and later rows of the periodic table, the d electrons in atoms of Main Groups III, IV, V, VI, and VII are counted as part of the core. Thus  

Ions are obtained by adding or removing electrons. The charge on an ion is given by 

An ionic compound is indicated by writing the Lewis structures for the two ions. 

A covalent bond model is constructed by allowing atoms to share pairs of electrons. Ordinarily, a shared pair is designated by a line  

Concepts Skills to Review Before You Study This Chapter 

B Relative magnitudes of atomic properties within a period 

In covalent bonding, two atoms share an electron pair localized between their nuclei (shown here as a bond line). Most covalent substances consist of individual molecules, each made from two or more atoms. C, In metallic bonding, many metal atoms pool their valence electrons to form a delocalized electron sea that holds the metal-lon cores together. 

It s important to remember that there are exceptions to these idealized bonding models in the world of real substances. For instance, all binary ionic compounds contain a metal and a nonmetal, but all metals do not form binary ionic 

Lewis Electron-Dot Symbols Depicting Atoms in Chemical Bonding 

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I. N. Levine, Quantum Chemistry Eourth Edition Prentice Hall, Englewood Cliffs (1991). W. H. E. Schwartz, Theoretical Models of Chemical Bonding Z. B. Maksic, Ed., Springer-Verlag, Berlin (1990). 

The covalent, or shared electron pair, model of chemical bonding was first suggested by G N Lewis of the University of California m 1916 Lewis proposed that a sharing of two electrons by two hydrogen atoms permits each one to have a stable closed shell electron configuration analogous to helium... 

An interesting discussion of these matters appears in Z.B. Maksic, On the Significance of Theoretical Models of Chemical Bonding Croat. Chem. Acta 1984, 57, I—III. 

The Lewis stmcture of a molecule shows how its valence electrons are distributed. These stmctures present simple, yet information-filled views of the bonding in chemical species, hi the remaining sections of this chapter, we build on Lewis stmctures to predict the shapes and some of the properties of molecules. In Chapter 10. we use Lewis stmctures as the starting point to develop orbital overlap models of chemical bonding. 

Z. Maksic, "Theoretical Models of Chemical Bonding" vol 1, Springer, Berlin (1991)... 

Mezey, P.G. (1998) Chemical bonding in proteins and other macromolecules. In Pauling s Legacy Modern Modelling of Chemical Bonding, Maksic, Z. and Orville-Thomas, J. (Eds.), Elsevier Science Publ., Amsterdam, The Netherlands. 

In the 1920s it was found that electrons do not behave like macroscopic objects that are governed by Newton s laws of motion rather, they obey the laws of quantum mechanics. The application of these laws to atoms and molecules gave rise to orbital-based models of chemical bonding. In Chapter 3 we discuss some of the basic ideas of quantum mechanics, particularly the Pauli principle, the Heisenberg uncertainty principle, and the concept of electronic charge distribution, and we give a brief review of orbital-based models and modem ab initio calculations based on them. 

E. Honegger and E. Heilbronner, The Equivalent Orbital Model and the Interpretation of PE Spectra, in Theoretical Models of Chemical Bonding, Part 3 (Ed. Z. B. Maksic), Springer-Verlag, Berlin, 1991, p. 99. 

Tapia, O. (1992) Theoretical evaluation of solvent effects,in Maksic, Z. B.(eds.), Theoretical models of chemical bonding, Spinger-Verlag, Berlin,pp.43.5-458. 

Given the ubiquitous character of molecular orbital concepts in contemporary discourse on electronic structure, ionization energies and electron affinities provide valuable parameters for one-electron models of chemical bonding and spectra. Electron binding energies may be assigned to delocalized molecular orbitals and thereby provide measures of chemical reactivity. Notions of hardness and softness, electronegativity,... 

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 set of all observed structures is necessarily a highly biased selection of all conceivable structures, but any proper model of chemical bonding in inorganic solids should be able to account for the structures that do not exist as well as for those that do. 

Our current model of chemical bonds is based on the work of the American chemist G. N. Lewis, one of the greatest of all chemists. It is even more remarkable that his insights were developed in 1916, before the electronic structures of atoms were understood. 

Jameson, C. J. In Theoretical Models of Chemical Bonding Maksic, Z. B., Ed. Molecular Spectroscopy, Electronic Structure, and Intramolecular Interactions, Part 3 Springer-Verlag Berlin, 1991, pp. 457-519. 

Fig. 38. Bond types. Electron-domain models of chemical bonds...

V. Magnasco and R. McWeeny. In Z.B. Maksic (ed.) Theoretical Models of Chemical Bonding, Part 2, Springer, Berlin Heidelberg New York, 1991. 

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