Solids ionic

Ionic solids are stable, high-melting substances held together by the strong electrostatic forces that exist between oppositely charged ions. The principles governing the structures of ionic solids were introduced in Section 8.5. In this section we will review and extend these principles. 

There are three types of holes in closest packed structures  

Trigonal holes are formed by three spheres in the same layer [Fig. 10.35(a)], 

Tetrahedral holes are formed when a sphere sits in the dimple of three spheres in an adjacent layer [Fig. 10.35(b)]. 

Octahedral holes are formed between two sets of three spheres in adjoining layers of the closest packed structures [Fig. 10.35(c)], 

To = average ionic bond length e = electronic charge (1.602 x 10 C) 

Ionic solids are only soluble in extremely polar solvents, due to dipole-dipole interactions between component ions and the solvent. Since the lattice energy of the crystal must be overcome in this process, the solvation of the ions (i.e., formation of [(H20) Na] ) represents a significant exothermic process that is the driving force for this to occur. 

Normally, ionic solids have very low conductivities. An ordinary crystal like sodium chloride must conduct by ion conduction since it does not have partially filled bands (metals) or accessible bands (semiconductors) for electronic conduction. The conductivities that do obtain usually relate to Ihe defects discussed in the previous section. The migration of ions may be classified into Ihree types. 

Vacancy mechanism. If there is a vacancy in a lattice, it may be possible for an adjacent ion of the type that is missing, normally a cation, to migrate into it, the difficulty of migration being related to the sizes of Ihe migrating ion and the ions that surround it and tend to impede it. 

interstitial mechanism. As we have seen with regard to Frenkel defects, if an ion is small enough (again, usualy a cation), it can occupy an interstitial site, such as a tetrahedral hole in an octahedral lattice. It may then move to other interstitial sites. 

Interstitialcy mechanism. This mechanism is a combination of the two above. It is a concerted mechanism, with one ion moving into an interstitial site and another ion moving into the vacancy thus created. These three mechanisms are shown in Fig. 7.15. 

In purely ionic compounds, the conductivity from these mechanisms is intrinsic and relates only to the entropy-driven Boltzmann distribution the conductivity will thus increase with increase in temperature. Because the number of defects is quite limited, the conductivities are low, of Ihe order of 10 6 II 1 cm-1. In addition, extrinsic vacancies will be induced by ions of different charge (see page 264). There exisl, however, a few ionic compounds that as solids have conductivities several orders of magnitude higher. One of the first to be studied and the one with the highest room-temperature conductivity, 0.27 fl cm-1, is rubidium silver iodide, RbAg.lj. 1 

In purely ionic compounds, the conductivity fbom these mechanisms is intrinsic and relates only to the entropy-driven Boltzmann distribution the conductivity will thus increase with increase in temperature. Because the number of defects is quite 

Diffusion in ionically bonded solids is more complicated than in metals because site defects are generally electrically charged. Electric neutrality requires that point defects form as neutral complexes of charged site defects. Therefore, diffusion always involves more than one charged species.9 The point-defect population depends sensitively on stoichiometry for example, the high-temperature oxide semiconductors have diffusivities and conductivities that are strongly regulated by the stoichiometry. The introduction of extrinsic aliovalent solute atoms can be used to fix the low-temperature population of point defects. 

Intrinsic Crystal Self-Diffusion. A simple example of intrinsic self-diffusion in an ionic material is pure stoichiometric KC1, illustrated in Fig. 8.11a. As in many alkali halides, the predominant point defects are cation and anion vacancy complexes (Schottky defects), and therefore self-diffusion takes place by a vacancy mechanism. For stoichiometric KC1, the anion and cation vacancies are created in equal numbers because of the electroneutrality condition. These vacancies can be created 

The vacancy populations enter the expressions for the self-diffusivity of the K+ cations and Cl- anions. Starting with Eq. 7.52 and using the method that led to Eq. 8.19 for vacancy self-diffusion in a metal, 

A similar expression applies to Cl self-diffusion on the anion sublattice. 

Self-diffusion of Ag cations in the silver halides involves Frenkel defects (equal numbers of vacancies and interstitials as seen in Fig. 8.116). In a manner similar to the Schottky defects, their equilibrium population density appears in the diffusivity. Both types of sites in the Frenkel complex—vacancy and interstitial— may contribute to the diffusion. However, for AgBr, experimental data indicate that cation diffusion by the interstitialcy mechanism is dominant [4]. The cation Frenkel pair formation reaction is 

These solids are characterized by cationic and anionic species that are associated through electrostatic interactions. All purely ionic salts possess crystalline structures. 

The Madelung constant and Born exponent appearing in Eq. 1 are related to the specific arrangement of ions in the crystal lattice. The Madelung constant may be considered as a decreasing series, which takes into account the repulsions among 

We shall look at the principles behind this method by considering some examples. First we consider a simple ionic solid, then move to an example where some allowance for covalency is required. Finally we consider the modelling of organic molecules. 

The starting point for the application of molecular mechanics to ionic solids is similar to the starting point for lattice energy calculations. Indeed the method can be used to calculate lattice energies, but it is also used to study the effect of defects, the nature of crystal surfaces and properties of crystals. 

As for lattice energies, we start by placing the ions of the crystal on their lattice sites. 

Electrostatic attraction between the positively- and negatively-charged ions. 

The ions are assumed to be on their lattice sites with their formal charges, so that in NaCl, for example, we have an array of Na+ and Cl ions. The net interaction can be obtained by summing the interactions over all the pairs of ions, including not only the attraction between Na+ and Cl but also the repulsion between ions of the same sign. The net interaction decreases with distance but slowly so that it is difficult to obtain an accurate value. 

Which element, W or Au, has the greater number of electrons in antibonding orbitals Which one would you expect to have the higher melting point  

The sheet of atoms formed by the process described above is not the lowest energy structure that can be formed. If this original sheet is sandwiched between similar states such that each of the Na and Cl atoms becomes surrounded by atoms of the opposite sign then a true minimum will be observed. If this order structure cannot be formed, because the entropy (disorder) is high, then the ensemble of atoms will be in the melt or gaseous state. 

The total number and relative magnitudes of the Coulombic interactions and whether they are attractive or repulsive are taken into account by using a factor 

The Madelung constant takes into account the different Coulombic forces, both attractive and repulsive, that act on a particular ion in a lattice. In the NaCl lattice, six Cl atoms surround each Na atom. The coordination number —6 describes the number of atoms which surround the selected reference atom. X-ray analysis indicates that each atom is a distance of 281 pm from its nearest neighbour. To calculate the Madelung constant we consider the four unit cells that surround the selected reference atom. Firstly there are twelve Cl ions each at a distance a from the central ion, and the Cff ions repel one another. The distance a is related to r by the equation 

Next there are eight Na ions each at a distance b from the central Cl ion, giving rise to attractive forces. Distance b is related to r by the equation 

Further attraetive and repulsive interactions occur, but as the distance involved increases, the Coulombic interactions decrease. 

A FIGURE 11.52 Sodium Chloride Unit Cell The different colored spheres in this figure represent the different ions in the compound. 

I We examine a more sophisticated model for I bonding in metals in Section 11.13. 

A FIGURE 11.55 The Electron Sea Model In the electron sea model for metals, the metal cations exist in a sea of electrons. 

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Born-Haber cycle A thermodynamic cycle derived by application of Hess s law. Commonly used to calculate lattice energies of ionic solids and average bond energies of covalent compounds. E.g. NaCl ... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

These are ionic solids and can exist as the anhydrous salts (prepared by heating together sulphur with excess of the alkali metal) or as hydrates, for example Na2S.9HjO. Since hydrogen sulphide is a weak acid these salts are hydrolysed in water,... 

Allan N L, G D Barrera, J A Purton, C E Sims and M B Taylor 2000. Ionic Solids at High Temperatures and Pressures Ah initio, Lattice Dynamics and Monte Carlo Studies. Physical Chemistry Chemical Physics 2 1099-1111. 

Ion-exchange methods are based essentially on a reversible exchange of ions between an external liquid phase and an ionic solid phase. The solid phase consists of a polymeric matrix, insoluble, but permeable, which contains fixed charge groups and mobile counter ions of opposite charge. These counter ions can be exchanged for other ions in the external liquid phase. Enrichment of one or several of the components is obtained if selective exchange forces are operative. The method is limited to substances at least partially in ionized form. 

The stmctures of ionic solids may be accounted for quite accurately by the use of a eottlombie interaetion potential between neighbouring ion pairs together with a suitable ion-core repulsion. 

Reijnen, P.J.L. (1970) Nonstoichiometry and sintering in ionic solids, in Problems on Nonstoichiometry, ed. Rabenau, A. (North-Holland, Amsterdam) p. 219. 

Colourless ionic solid sublimes at 32.4° to unstable molecular gas (angle N-O-N -180°)... 

PCI5 is even closer to the ionic-covalent borderline than is PF5, the ionic solid [PCl4]" [PCl6] melting (or subliming) to give a covalent molecular... 

The overall lattice energies of ionic solids, as treated by the Born-Eande or Kaputin-sldi equations, thus depends on (i) the product of the net ion charges, (ii) ion-ion separation, and (iii) pacldng efficiency of the ions (reflected in the Madelung constant, M, in the Coulombic energy term). Thus, low-melting salts should be most... 

The scope of the term corrosion is continually being extended, and Fontana and Staehle have stated that corrosion will include the reaction of metals, glasses, ionic solids, polymeric solids and composites with environments that embrace liquid metals, gases, non-aqueous electrolytes and other non-aqueous solutions . 

When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

As pointed out in Chapter 2, when an ionic solid dissolves in water, the cations and anions separate from each other. This process can be represented by a chemical equation in which the reactant is the solid and the products are the positive and negative ions in water (aqueous) solution. For the dissolving of MgCl2, the equation is... 

Similar relationships hold for ionic solids containing polyatomic ions (Table 2.2) or transition metal cations (Figure 4.2). 

Bases, like acids, are classified as strong or weak. A strong base in water solution is completely ionized to OH- ions and cations. As you can see from Table 4.1, the strong bases are the hydroxides of the Group 1 and Group 2 metals. These are typical ionic solids, completely ionized both in the solid state and in water solution. The equations written to represent the processes by which NaOH and Ca(OH)2 dissolve in water are... 

Ionic solids do not conduct electricity because the charged ions are fixed in position. They become good conductors, however, when melted or dissolved in water. In both cases, in the melt or solution, the ions (such as Na+ and Cl-) are free to move through the liquid and thus can conduct an electric current. 

Solids with different structures, (a) Diamond, a network covalent solid, (b) Potassium dichromate. K2 2O7, an ionic solid, (c) Manganese, a metallic solid. 

Crystals have definite geometric forms because the atoms or ions present are arranged in a definite, three-dimensional pattern. The nature of this pattern can be deduced by a technique known as x-ray diffraction. Ihe basic information that comes out of such studies has to do with the dimensions and geometric form of the unit cell, the smallest structural unit that, repeated over and over again in three dimensions, generates the crystal In all, there are 14 different kinds of unit cells. Our discussion will be limited to a few of the simpler unit cells found in metals and ionic solids. 

The force of attraction between H20 molecules and the ions, which tends to bring the solid into solution. If this factor predominates, the compound is very soluble in water, as is the case with NaCl, NaOH, and many other ionic solids. 

The force of attraction between oppositely charged ions, which tends to keep them in the solid state. If this is the major factor, the water solubility is very low. The fact that CaC03 and BaS04 are almost insoluble in water implies that interionic attractive forces predominate with these ionic solids. 

A salt is an ionic solid containing a cation other than H+ and an anion other than OH or O2-. When a salt such as NaCl, K2CO3, or A1(N03)3 dissolves in water, the cation and anion separate from one another. 

Strategy You are given and enough information to determine Hi,ac and nNaLac-Because sodium lactate, like all ionic solids, is completely ionized in water, n - = MNaLac-... 

As we saw in Chapter 4, a precipitate forms when a cation from one solution combines with an anion from another solution to form an insoluble ionic solid. We also considered how to predict whether such a reaction would occur and, if so, how to represent it by a net ionic equation. 

When an ionic solid consists of anions and cations of different charges, the relation between Ksp and s takes a different form, but the principle is the same (Example 16.4). 

Frequently we find that the experimentally determined solubility of an ionic solid is larger than that predicted from /Qp. Consider, for example, PbCl2, where the solubility calculated from the relation... 

The effect illustrated in Example 16.6 is a general one. An ionic solid is less soluble in a solution containing a common ion than it is in water (Figure 16.3, p. 438). 

Many different methods can be used to bring water-insoluble ionic solids into solution. Most commonly, this is done by adding a reagent to react with either the anion or the cation. The two most useful reagents for this purpose are—... 

Column A lists a series of ionic solids, all of which are insoluble in water. Match each of these compounds with the appropriate description(s) listed in Column B. Note that more than one description can fit a particular compound. 

Shown below is a representation of the ionic solid MX, where M cations are represented by squares and X anions are represented by circles. Fill in the box after the arrow to represent what happens to the solid after it has been completely dissolved in water. For simplicity, do not represent the water molecules. 

Ionic radius The radius assigned to a monatomic ion, 154 main-group elements, 153t Ionic solids, 240-245 Ionization expression, 378q percent, 362... 

Salt An ionic solid containing any cation other than H+ and any anion other than OH" or O2-, 372 carboxylic acid, 595 pH, 373-374... 

These three solids, sodium chloride, calcium chloride, and silver nitrate are similar, hence they are classified together. They all dissolve in water to form aqueous ions and give conducting solutions. These solids are called Ionic solids. 

Silver chloride, like sodium chloride, is an ionic solid. 

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