Ionic solid typical properties

The strong attraction between oppositely charged ions accounts for the typical properties of ionic solids, such as their high melting points and their brittleness. A high temperature is required before the ions are able to move past one... 

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. 

Ionic interactions are electrostatic by nature, and occur between ions of opposite charge. The overwhelming majority of ionic compounds are solids, although a few biological exceptions do occur. Table 2.7 lists a few typical properties of ionic compounds. 

There are several methods you can use to predict the type of bond in an unknown substance. For example, you can consider the substance s physical properties. In contrast to ionic solids, covalent (molecular) compounds typically have the following properties ... 

Oxides exhibit similar trends. In the third row, for example, Na20 and MgO are typical high-melting, ionic solids, and P4O10, S03, and C1207 are volatile, covalent, molecular compounds (Section 14.9). The metallic or nonmetallic character of an oxide also affects its acid-base properties. Na20 and MgO are basic, for example,... 

The halides are all typically ionic solids, but can be vaporized as molecules, the structures of which are not all linear. On account of its dispersion and transparency properties, CaF2 is used for prisms in spectrometers and for cell windows (especially for aqueous solutions). It is also used to provide a stabilizing lattice for trapping lanthanide +2 ions. 

Let us then turn to the ionic solids themselves. Kim and Ciordon (1974) recalculated the total energy for the alkali halides that form rocksalt structures and thereby computed from first principles the lattice spacing, the separation energy (the energy per ion pair required to separate the solid into isolated ions -this comes from the theory more naturally than does the cohesive energy, which is relative to isolated neutral atoms), and the bulk modulus. For KCI, the agreement of the values for the three properties with experimental values is typical of the calculations. The calculated (and in parentheses, the experimental) values for KCI are 3.05 (3.15) A, 175 (166) kcal/mole, and 2.3 (1.9) x 10 dync/cmA Again we may say that the interactions are quite well understood in terms of the microscopic theory. We shall return to the interpretation of these properties in terms of simple models in Section 13-D. 

Recall that the chemical property most characteristic of a metal is the ability to lose its valence electrons. The Group 1A elements are very reactive. They have low ionization energies and react readily with nonmetals to form ionic solids. A typical example involves the reaction of sodium with chlorine to form sodium chloride,... 

We classify crystalline solids into categories according to the types of particles in the crystal and the bonding or interactions among them. The four categories are (1) metallic solids, (2) ionic solids, (3) molecular solids, and (4) covalent solids. Table 13-10 summarizes these categories of solids and their typical properties. 

For ionomers, the ratio n/m typically does not exceed 10 mol%. The metal ions act as the cross-links between chains. The ionic interchain forces confer solid-state properties normally associated with a cross-linked structure on ionomers, but the cross-links are labile at processing temperatures. Consequently, ionomers, like other thermoplastic materials, can be processed on conventional molding and extrusion equipment. 

The explanations of such solid-state phenomena involve many factors. Nevertheless, it is known that covalent energy terms are of underlying importance in determining many of these properties of ionic solids as well as the behavior of molecular complexes. Although, therefore, we cannot draw a neat boundary between typically ionic compounds and, on the one hand, complex compounds nor, on the other, covalent solids and metals, many properties of compounds to which the ionic model may be applied indicate that a more rigorous explanation of the bonding is necessary. 

These properties indicate that tire atoms are capable of slipping witii respect to one another. Ionic solids or crystals of most covalent compounds do not exhibit such behavior. These types of solids are typically brittle and fracture easily. Consider, for example, the difference between dropping an ice cube and a block of aluminum metal onto a concrete floor. 

Table 12.1 Bond length relaxation for typical covalent, metallic, and ionic solids and its effect on the physical properties of the corresponding solid or surface...

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