Simple ionic compounds

The compounds which according to almost any theory should lie the most nearly ionic in character are the alkali halides. It is, therefore, particularly interesting to compare their structures with those predicted by our simple theory. We see in Table II that only the NaCl rocksalt (Fig. 4) 

Radius Ratios and Crystal Structures of Alkali Halides (ISO). 

Where Two Structures are Listed, the First Represents the Stable Modification 

A study of a number of other compounds which are generally considered to be among the most completely ionic shows that cubic coordination is not confined to the alkali halides. It occurs, for example, in the fluorite structure in which many ionic MXa compounds crystallize. On the other hand the isolated [Tab s]3 group has the square antiprism arrangement (64) predicted by the simple theory. This underlines a point of great importance to the understanding of ionic crystal structures, namely, that the requirement that a structure can be extended indefinitely in space imposes severe restrictions on the types of coordination which are possible. Cubic coordination can be extended indefinitely, but it is not possible to form an extended 

To proceed further with our comparison of theory with experiment we require values of the radii of the ions concerned. The values recommended by Pauling (114) arc given in Table III and a discussion of their derivation in the Appendix. 

FIGURE 6.3 Lewis structures of the chloride and oxide monatomic ions. 

FIGURE 6.4 Lewis structures of the sodium, magnesium, and aluminum monatomic ions. 

FIGURE 6.5 Lewis structures of several simple ionic compounds. 

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The enthalpy of solution is quite small for many simple ionic compounds and can be either positive or negative. It is the difference between two large quantities, the sum of the hydration enthalpies and the lattice energy. 

Prediction of solubility for simple ionic compounds is difficult since we need to know not only values of hydration and lattice enthalpies but also entropy changes on solution before any informed prediction can be given. Even then kinetic factors must be considered. 

Coordination compounds are named in much the same way as simple ionic compounds. The cation is named first, followed by the anion. Examples include... 

In the preceding chapter we looked at the elements of the third row in the periodic table to see what systematic changes occur in properties when electrons are added to the outer orbitals of the atom. We saw that there was a decided trend from metallic behavior to nonmetallic, from base-forming to acid-forming, from simple ionic compounds to simple molecular compounds. These trends are conveniently discussed... 

The systematic investigation of the chemistry of the transition elements began in the nineteenth century, and it rapidly became apparent that many of the compounds were somewhat different from those with which chemists were then familiar. There was a clear difference between the behaviour of simple ionic compounds such as sodium chloride and typical transition-element compounds such as FeCl2-4H20. It was also obvious that the compounds did not resemble the typically covalent compounds of organic chemistry. It was considered that many of the compounds formed by transition metals were of a complex constitution, and they were accordingly known as complexes. 

The structures of ionic compounds comprising complex ions can in many cases be derived from the structures of simple ionic compounds. A spherical ion is substituted by the complex ion and the crystal lattice is distorted in a manner adequate to account for the shape of this ion. 

Writing the formula for ionic compounds requires us to know the charge of the cation and anion making up the ionic compound. Information on the charge of common ions can be obtained from the periodic table. More will be said about this in Chapter 7, but for now, a few basic rules will help us write the formulas for simple ionic compounds ... 

As the valency of the metal increases, the bonding in these simple binary compounds becomes more covalent and the highly symmetrical structures characteristic of the simple ionic compounds occur far less frequently, with molecular and layer structures being common. Many thousands of inorganic crystal structures exist, ffere we describe just a few of those that are commonly encountered and those that occur in later chapters. 

Nciming a simple ionic compound is easy. You pair the name of the cation with the neune of the anion and then change the ending of the anion s ncime to -ide. The cation always precedes the anion in the final name. For example, the chemical name of NaCl (a compound made up of one sodium atom and one chlorine atom) is sodium chloride. 

Clearly, U is the biggest number in the cycle and is the main driving force for the formation of ionic compounds. Nevertheless, the other factors can tip the balance one way or another. For example, AHSub is particularly large for the transition metals niobium, tantalum, molybdenum, tungsten, and rhenium, with the result that, in their lower oxidation states, they do not form simple ionic compounds such as ReCl3 but rather form compounds that contain clusters of bonded metal atoms (in this example, Re3 clusters are involved, so the formula is better written ResClg). 

The first four structures described below contain equal numbers of cations and anions, that is, the 1 1 and 2 2 salts. Most simple ionic compounds with such formulations crystallize in one of these four structures. They differ principally in the coordination number, that is. the number of counterions grouped about a given ion, in these examples four, six, and eight... 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

Although not many simple ionic Compounds have been studied with the requisite accuracy to provide data on ionic radii, there are enough to provide a basis for a complete set of ionic radii. Such a set has been provided in the crystal radii of Shannon and Prewitt.19 Values of these radii are given in Table 4.4. 

Suppose that someone argues with you that your answer to Problem 432 is invalid, and that any prediction that Neil Bartlett might have made on the basis of similar reasoning (See Chapter 17) is equally invalid—he was just lucky—the reaction product 0/ Eq. 433 is not a simple ionic compound, XeH PtFg, but a mixture of compounds, and apparently the xenon is covalently bound. What Is your reply ... 

Realizing that these formulations implied a preuse statement of the number of ions formed in solution. Werner chose as one of his first experimental studies measurement of the conductivities of a large number of coordination compounds. 1 Some of the results of this work are listed in Table I l.l together with values for simple ionic compounds for comparison. 

In all of these compounds, even the tetrahedral ones, a possible starting point for the calculation of properties is an ionic electronic structure with the effects of interatomic matrix elements treated in perturbation theory. As wc liave indiettted, and as will be seen in detail in the next section, it is even possible to treat tlic polar covalent nontransition-metal solids in this way. Thus we should be able to calculate properties of the transition-metal compounds just as we did for the simple ionic compounds. 

Occurrence of Ionic Simple ionic compounds form only between very active metallic elements and very Bonding active nonmetals. Two important requisites are that the ionization energy to form... 

The Structures of Chapter 4 considered the topic of simple ionic compounds such as NaCl, CsCl, Cap2,... 

In dilute aqueous solutions, it has been demonstrated experimentally for poorly soluble ionic salts (solubilities less than 0.01 molL ) that the mathematical product of the total molar concentrations of the component ions is a constant at constant temperature. This product, is called the solubility product. Thus for a saturated solution of a simple ionic compound AB in water, we have the dynamic equilibrium ... 

Compounds containing coordination complexes are named following the same rules as those for simple ionic compounds The positive ion is named first, followed (after a space) by the name of the negative ion. 

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