Describing Ionic Bonds

Metallic bonding, seen in sodium and other metals, represents another important type of bonding. A crystal of sodium metal consists of a regular arrangement of sodium atoms. The valence electrons of these atoms move throughout the crystal, attracted to the positive cores of all Na ions. This attraction holds the crystal together. 

What determines the type of bonding in each substance How do you describe the bonding in various substances In this chapter we will look at some simple, useful concepts of bonding that can help us answer these questions. We will be concerned with ionic and covalent bonds in particular. 

Natural crystals of sodium chloride mineral (halite). 

To understand why ionic bonding occurs, consider the transfer of a valence electron from a sodium atom (electron configuration [Ne]3s ) to the valence shell of a chlorine atom ([Ne]3s 3p ). You can represent the electron transfer by the following equation  

Lewis Electron-Dot Symbols for Atoms of the Second and Third Periods 

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As is the case with ionization potential, the electron affinity is a useful property when considering the chemical behavior of atoms, especially when describing ionic bonding, which involves electron transfer. 

Sq can be calculated from the theoretically derived U(r) curves of the sort described in Chapter 4. This is the realm of the solid-state physicist and quantum chemist, but we shall consider one example the ionic bond, for which U(r) is given in eqn. (4.3). Differentiating once with respect to r gives the force between the atoms, which must, of course, be zero at r = rg (because the material would not otherwise be in equilibrium, but would move). This gives the value of the constant B in equation (4.3) ... 

If you were given a sample of a white solid, describe some simple experiments that you would perform to help you decide whether or not the bonding involved primarily covalent bonds, ionic bonds, or van der Waals forces. 

Plutonium cations in whatever oxidation state can be described as hard acids and interact with anionic species by ionic bonding. As a result certain generalizations can be made about the relative complexing tendencies of the different oxidation states. 

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 ... 

Figure 5.1 depicts the distance dependence of this potential energy and that of other interactions described in the following four sections. These interactions are summarized in Table 5.1 notice that the energies of these interactions are much lower than the energies typical of ionic bonds. 

The role of oxygen in these inorganic polymers in important. In Zacheriasen s theory, oxygen was described as bridging if part of a covalent structure (10.4). When there was an ionic bond, the oxygen was described as non-bridging (10.5). 

Lewis considered covalent and ionic bonds to be two extremes of the same general type of bond in which an electron pair is shared between two atoms contributing to the valence shell of both the bonded atoms. In other words, in writing his structures Lewis took no account of the polarity of bonds. As we will see much of the subsequent controversy concerning hypervalent molecules has arisen because of attempts to describe polar bonds in terms of Lewis structures. 

This was averaged over the total distribution of ionic and dipolar spheres in the solution phase. Parameters in the calculations were chosen to simulate the Hg/DMSO and Ga/DMSO interfaces, since the mean-spherical approximation, used for the charge and dipole distributions in the solution, is not suited to describe hydrogen-bonded solvents. Some parameters still had to be chosen arbitrarily. It was found that the calculated capacitance depended crucially on d, the metal-solution distance. However, the capacitance was always greater for Ga than for Hg, partly because of the different electron densities on the two metals and partly because d depends on the crystallographic radius. The importance of d is specific to these models, because the solution is supposed (perhaps incorrectly see above) to begin at some distance away from the jellium edge. 

In some cases, elements having electronegativities too low to give ionic bonding with hydrogen also tend to be unreactive, so that direct combination of the elements is not feasible. In such cases, the procedure just described can be used to prepare the hydride. For example, silicon hydride, SiH4 (known as silane), can be produced by the reactions... 

The first two terms in equation.(3) describe a spherical deformation being characteristic for the ionic bonding and is known in the precise X-ray structure analysis as K-model ... 

One ionic bond that often helps establish tertiary structure is a disulfide bond between two cysteine side chain groups—for instance, in the enzyme lysozyme as shown in Figure 2.10. Lysozyme is not a metalloprotein, such as will be studied in this text, but it is a small enzyme and is illustrative of some secondary and tertiary structures found in the more complex molecules described in the following chapters. Lysozyme protects biological species from... 

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