More Complicated Ionic Compounds

We are now ready to draw Lewis electron-dot structures of ionic cointounds that include at least one polyatomic ion. The polyatomic ions in such structures are drawn according to the steps in Table 6.1. The total structure is then assembled as was done for the other ionic compounds that were discussed in Section 6.3. 

Draw the Lewis electron-dot structure of sodium sulfate (NajSOJ. Solution 6.6 

Two sodium ions, Na+, will be shown with one sulfate ion. We draw the sulfate ion according to the procedure in Table 6.1. 

FIGURE 6.19 An example of an ionic compound sodium sulfate (Na2S04) that includes a polyatomic ion, the sulfate ion SO42-. The two sodium ions are shown and then the Lewis structure of the sulfate ion must be determined, (a) After performing step 2, including the brackets and charge, (b) The complete structure after performing step 4 for the sulfate ion. 

Draw the Lewis electron-dot structure of ammonium phosphate 

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In simple ionic compounds, each ion only occupies one type of environment, with all the ions of the same type having exactly the same geometric relationship to all the other ions in the crystal lattice. In more complicated ionic compounds, it is possible for ions of one species to occupy one of a limited number of environments, but this is the exception rather than the rule at this level. 

Several additional, more complicated structure types are known for ionic compounds. For example, according to the radius ratio, one could expect the rutile type for strontium iodide (rSr2+ /i = 0.54). In fact, the structure consists of Sr2+ ions with a coordination number of 7 and anions having two different coordination numbers, 3 and 4. 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

It can be seen from Table 1 that the compounds LiF and NaF have almost pure ionic character. Magnesium oxide MgO, however, has a more complicated bonding character with considerable contribution of the covalent component. 

The synthesis of non-ionic contrast agents is much more complicated. Due to the higher aqueous solubility of the intermediates, purification processes have to be more sophisticated than for the ionic compounds. Moreover, for the carboxylic acid, salt purification procedures of ionic substances are no longer available for the non-ionic derivatives. Now, recrystallisation. 

The simple theory of the heteropolar bond was developed rapidly in contrast to the theory of the homopolar bond where great difficulties were encountered. Nevertheless, in the last decades important advances have been made, but the enormous mathematical difficulties encountered have resulted in the strict theory being applied only to the simplest examples of chemical combination. The theory of the ionic bond has no difficulties of a mathematical kind and in consequence can be used for more complicated compounds. In the following pages this theory will be treated first, and later a very elementary, schematic presentation of the theory of the homopolar bond will be given. 

In order to fully appreciate and understand molecular structure, you will need to be able to construct representations of various molecules. One of the easiest ways to do this is using Lewis structures. The procedure is a bit more complicated than for ionic compounds because of the increasing complexity of covalent compounds. The basic procedure for constructing Lewis diagrams of molecules consists of 4 steps ... 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

Whereas ionization and dissociation are clearly defined processes on the left side of Scheme 35, the situation is more complicated for carbanionic systems (Scheme 35, right). Organic alkali metal compounds, for example, which often exist as aggregates, are often described as covalent species with a certain percentage of ionic character [140-142]. If the formal carbanion is a resonance-stabilized species (e.g, diphenylmethyl lithium or sodium), the species with the closest interaction between the organic fragment and the metal is usually called a contact ion pair. In... 

There are some important exceptions to the rules discussed here. For example, tin forms both Sn2+ and Sn4+ ions, and lead forms both Pb2+ and Pb4+ ions. Also, bismuth forms Bi3+ and Bi5+ ions, and thallium forms Tl+ and Tl3+ ions. There are no simple explanations for the behavior of these ions. For now, just note them as exceptions to the very useful rule that ions generally adopt noble gas electron configurations in ionic compounds. Our discussion here refers to representative metals. The transition metals exhibit more complicated behavior, forming a variety of ions that will be considered in Chapter 20. 

For covalent solids, by far the major contribution to the dielectric constant results from electronic polarization. In ionic solids, the situation is more complicated, as discussed below. It should also be pointed out that the electronic polarizability of a compound can, to a very good approximation, be taken as the sum of the polarizabilities of the atoms or ions making up that compound. 

Alkali halides are compounds with a strong ionic character and without a solvent they are stabilized by the strong electrostatic interaction between the cation and the anion. In the present study we limit attention to the dissociation potentials comparing the curves obtained for the free molecule and for the molecule in water solution. The solvent is treated only as a continuum medium and then in this way we cannot consider the formation of complexes between the ions and the water molecules which instead are extensively studied by means of Monte Carlo (MC) and molecular dynamics (MD) simulations (see for example [13]). Although it could be possible to include some water molecules with the sodium chloride as a more complicated solute, we have preferred to focus attention on the solvent effect on the electronic structure of the simplest solute, this effect being the most important in the next two examples. 

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