Ionic compounds illustration

This discussion of the energies involved in the formation of solid ionic compounds illustrates that a variety of factors operate to determine the... 

The composition of an ionic compound is determined by overall charge neutrality. The total positive charge on the cations must exactly balance the total negative charge on the anions. The following names and formulas of ionic compounds illustrate this point ... 

When three or more different elements occur in a compound, the order depends on whether or not the compound contains ions. We describe ionic compounds in Section 34. Many multiple-element compounds that do not contain ions contain carbon. The formulas of carbon-containing compounds start with carbon, followed by hydrogen. After that, any other elements appear in alphabetical order, as illustrated by the following examples C2 He O, C4 H5 BrO, CH3 Cl, and Cg Hio N4 O2. 

Here are some specific examples illustrating chemical formulas of ionic compounds. 

This outstanding behavior of bis(pentamethylcyclopendienyl)stannylene has been explained by the energetically favorable formation of the ionic compound 79 which contains the 6-membered cluster C5Sn 174>. The structure of the boron tetrafluoride compound is illustrated in Fig. 15 the tin atom in the cation is located at the apex of a pentagonal pyramide 173). 

Results have shown that the properties of solids can usually be modeled effectively if the interactions are expressed in terms of those between just pairs of atoms. The resulting potential expressions are termed pair potentials. The number and form of the pair potentials varies with the system chosen, and metals require a different set of potentials than semiconductors or molecules bound by van der Waals forces. To illustrate this consider the method employed with nominally ionic compounds, typically used to calculate the properties of perfect crystals and defect formation energies in these materials. 

Another way of deriving the formula of the ionic compound is the crisscross rule. In this technique the cation and anion are written side by side. The numerical value of the superscript charge on the cation (without the sign) becomes the subscript on the nonmetal in the compound, and the superscript charge on the anion becomes the subscript on the metal in the compound. Figure 11.3 illustrates the crisscross rule for the reaction between aluminum and oxygen. 

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

Ionic conductors are ionic compounds. Therefore it is appropriate to start with ions rather than atoms to construct the electronic energy level diagrams. Fig. 3.2 illustrates such a construction for the electronic and ionic insulator MgO. The energy levels and 0 correspond... 

Many organometallic catalysts are soluble in ionic liquids, especially including ionic compounds. Neutral species, such as Wilkinson s catalyst, are also soluble to some extent in ionic liquids (169). There are numerous examples illustrating the dispersion and isolation of organometallic catalysts in ionic liquids a list of examples is given in a recent review (/). 

Ionic compounds are usually precipitated in the presence of an electrolyte. To understand why, we must discuss how tiny colloidal crystallites coagulate (come together) into larger crystals. We illustrate the case of AgCI, which is commonly formed in 0.1 M HN03. 

So what happens to polar molecules, such as water molecules, when they are near an ionic compound, such as sodium chloride The opposite charges electrically attract one another. The positive sodium ions attract the negative side of the water molecules, and the negative chloride ions attract the positive side of the water molecules. This is illustrated in Figure 7.1. Such an attraction between an ion and the dipole of a polar molecule is called an ion—dipole attraction. 

Even if there are exceptions to the radius ratio rule, or if exact data are hard to come by, it is still a valid guiding principle. Cite three independent examples of pairs of compounds illustrating structural differences resulting from differences in ionic radii. 

The use of Eq. 4.13 to predict the lattice energy of an ionic compound may be illustrated as follows. For sodium chloride the various factors are... 

For polyatomic molecules, it is important to distinguish between a polar molecule and a polar bond. Although each bond in a polyatomic molecule may be polar, the molecule as a whole will be nonpolar if the dipoles of the individual bonds cancel one another. For example, the two 8+C—O8- dipoles in carbon dioxide, a linear molecule, point in opposite directions, so they cancel each other (30). As a result, C02 is a nonpolar molecule even though its bonds are polar. The electrostatic potential diagram (31) illustrates this conclusion. In contrast, the two 8-0—H8+ dipoles in H20 lie at 104.5° to each other and do not cancel, so H20 is a polar molecule (32). This polarity is part of the reason why water is such a good solvent for ionic compounds. 

For ionic compounds the coordination number is the number of anions that are arfang d about the cation in a organized structure. For example, NaCl has a coordination number of 6. In otherwords, 6 CF atoms surround 1 Na+ atom. The number of anions that can surround a cation is dependent (but not entirely) on the relative sizes of the ions involved. Table 2.15 illustrates thtnmlios of the radii of the ions and their coordination number. 

In general, the solubility of a slightly soluble ionic compound is decreased by the presence of a common ion in the solution, as illustrated in Figure 16.11. The quantitative aspects of the common-ion effect are explored in Worked Example 16.11. 

Many ionic compounds have structures based on either fee or hep packing of one of the ions. Both of these structures have sites of fourfold coordination and sites of sixfold coordination. Several simple structures for AB compounds are illustrated in Figure 13.12. Both the zinc blende structure, which is based on... 

As has been pointed out previously, ionic compounds are characterized by a Fermi level EF that is located within an s-p-state energy gap Ef. It is for this reason that ionic compounds are usually insulators. However, if the ionic compound contains transition element cations, electrical conductivity can take place via the d electrons. Two situations have been distinguished the case where Ru > Rc(n,d) and that where Rlt < Rc(n,d). Compounds corresponding to the first alternative have been discussed in Chapter III, Section I, where it was pointed out that the presence of similar atoms on similar lattice sites, but in different valence states, leads to low or intermediate mobility semiconduction via a hopping of d electrons over a lattice-polarization barrier from cations of lower valence to cations of higher valence. In this section it is shown how compounds that illustrate the second alternative, Rtt < 72c(n,d), may lead to intermediate mobility, metallic conduction and to martensitic semiconductor metallic phase transitions. 

A Type I binary ionic compound contains a metal that forms only one type of cation. The rules for naming Type I compounds are illustrated by the following examples ... 

To illustrate the principles of electron configurations in stable, solid ionic compounds, we will consider the formation of an ionic compound from calcium and oxygen. We can predict what compound will form by considering the valence electron configurations of the two atoms ... 

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