Bonding ionic, 10, Chapter

As discussed in Chapter 1, chemists have long recognized two types of bonds ionic and covalent. However, a purely ionic bond is a hypothetical concept because in any bond there is... 

When spherical objects are stacked to produce a three-dimensional array (crystal lattice), the relative sizes of the spheres determine what types of arrangements are possible. It is the interaction of the cations and anions by electrostatic forces that leads to stability of any ionic structure. Therefore, it is essential that each cation be surrounded by several anions and each anion be surrounded by several cations. This local arrangement is largely determined by the relative sizes of the ions. The number of ions of opposite charge surrounding a given ion in a crystal is called the coordination number. This is actually not a very good term because the bonds are not coordinate bonds (see Chapter 16). For a specific cation, there will be a limit to the number of anions that can surround the cation because... 

A number of different molecular mechanisms can underpin the loss of biological activity of any protein. These include both covalent and non-covalent modification of the protein molecule, as summarized in Table 6.5. Protein denaturation, for example, entails a partial or complete alteration of the protein s three-dimensional shape. This is underlined by the disruption of the intramolecular forces that stabilize a protein s native conformation, namely hydrogen bonding, ionic attractions and hydrophobic interactions (Chapter 2). Covalent modifications of protein structure that can adversely affect its biological activity are summarized below. 

Chapter 5 discusses the way that anions (atoms with negative chcirge) and cations (atoms with positive charge) attract one another to form ionic bonds. Ionic bonds hold together ionic compounds. The anions and cations in a given ionic compound are important factors in how you name that compound. 

In this chapter, we explored two types of chemical bonds ionic and covalent. Ionic bonds are formed when one or more electrons move from one atom to another. In this way, the atoms become ions—one positive, the other negative—and are held together by the resulting electrical attraction. Covalent bonds form when atoms share electrons. When the sharing is completely equitable, the bond is nonpolar covalent. When one atom pulls more strongly on the electrons because of its greater electronegativity, the bond is polar covalent and a dipole may be formed. 

Jprgensen57 has referred to this tendency of fluoride ions to favor further coordination by a fourth fluoride (the same is true for hydrides) as symbiosis." Although other factors can work to oppose the symbiotic tendency, it has widespread effect in inorganic chemistry and helps to explain the tendency for compounds to be symmetrically substituted rather than to have mixed substituents. We have seen (Chapter 5) that the electrostatic stabilization of C—F bonds (ionic resonance energy) will be maximized in CF4, and similar arguments can be made for maximizing hard-hard or soft-soft interactions. 

As we discussed in chapter 2, there are two limiting kinds of chemical bonds ionic bonds and covalent bonds. The overarching driving force in formation of... 

The list makes it clear that a bar of copper or iron has properties that are entirely different from substances held together by ionic or covalent bonds. This chapter aims to show that the source of these properties is the metallic bond. But what kind of bonding would make metals dense and conduct electricity readily What sort of structure would make them lustrous and malleable Why should metals eject electrons when a light is shined on them ... 

From the experimental values of dipole moments it is possible, in a number of cases, to make a semi-quantitative evaluation of the weights of the various valence bond structures contributing to a bond (see Chapter 18). These calculations must be regarded as only approximate since the bond is described in terms of the Heider-London theory with the superposition of ionic states. The results cannot, therefore, be more precise than is permitted by the Heitler-London approximation. Nevertheless, the calculations are of significance since they permit an assessment to be made of the more important structures contributing to the bond and thus assist in predicting and explaining the reactivity of bonds. 

The type of bonding found in metals is quite different from that in other crystals. As we compare the various main group and transition metals in the periodic table we see only small differences in electronegativity. So, there is little tendency for ionic bonding in metals. The electronic configurations of metal atoms, even in the transition metals, do not have nearly-filled subshells, so there is little tendency to form covalent bonds by sharing electrons to achieve a stable octet. The familiar classical models of chemical bonding (see Chapter 3) do not extend to metals. 

This chapter provides a substantial introduction to molecular structure by coupling experimental observation with interpretation through simple classical models. Today, the tools of classical bonding theory—covalent bonds, ionic bonds, polar covalent bonds, electronegativity, Lewis electron dot diagrams, and VSEPR Theory—have all been explained by quantum mechanics. It is a matter of taste whether to present the classical theory first and then gain deeper insight from the... 

It is important to be able to tell whether a substance is ionic, nonpolar covalent, or polar covalent. You should review the discussion of bonding in Chapters 7 and 8. 

Chapter 4 describes how the Chemical Properties of the Elements are related to their valence shell configuration, i.e. characteristic or group oxidation number, variable valence, ionic and covalent bonding. This chapter includes a section on the volumetric calculations used in an introductory inorganic practical course, including the calculation of the stoichiometry factors for chemical reactions. 

By no means, this short chapter could include all the types of hydrogen-bonded materials, or all types of transformations of hydrogen bonds. It was only aimed to present very briefly a few selected examples, and only one structural approach to the structural transformations has been presented. Some of the important classes of hydrogen-bonded materials have not been even mentioned, for example the hydrogen-bonded ionic conductors [38-40]. 

The active center involved in the propagation reaction may be a free-radical, ion, or metal-carbon bond (see Chapters 6-10). A propagating species will be more stable if the unpaired electron or ionic charge can... 

Pericyclic reactions656,657 are the second distinct class of the three, more or less exclusive categories of organic reactions—ionic (Chapters 4 and 5), pericyclic (this Chapter) and radical (Chapter 7). Their distinctive features are that they have cyclic transition structures with all the bond-making and bond-breaking taking place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. 

QUESTIONS ADDRESSED IN THIS CHAPTER AND IN CHAPTER 10. WE BEGIN BY LOOKING AT THE TWO TYPES OF BONDS------IONIC AND COVALENT------------------------------------------AND THE FORCES THAT STABILIZE THEM. 

In chapter 1, we discussed rather briefly in terms of Bohr orbits, two types of chemical bond—ionic and covalent. We saw that the essential picture was that of electron transfer in an ionic bond, and a sharing of electrons in a covalent bond. 

In this chapter we have been looking at three types of chemical bonds covalent bond, ionic bonds and metallic bonds. The bonds are described by using different models and theoiy which introduce the molecular orbitals. These molecular orbitals are formed from atomic orbitals which we heard about in chapter 1. 

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