Ionic and molecular solids

On page 10 you saw that solids are made of particles packed closely together, in a regular pattern. If the particles are ions, the solids are called ionic solids. If they are molecules, the solids are called molecular or covalent solids. The two types of solids have quite different properties, as you will see below. 

One of the most common ionic solids is sodium chloride—it is ordinary table salt. You already know quite a lot about its structure  

Sodium chloride is made of sodium and chloride ions, packed in a regular pattern. This arrangement is called a lattice. The ions are held togedier by strong ionic bonds. 

The pattern repeats millions of times. The result is a piece of solid with straight edges and flat faces, called a crystal. Above is a crystal of sodium chloride, magnified 35 times. 

The crystals look white and shiny. A box of table salt contains millions of them. 

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Near room temperature most gases become less soluble in water as the temperature is raised. The lower solubility of gases in warm water is responsible for the tiny bubbles that appear when cool water from the faucet is left to stand in a warm room. The bubbles consist of air that dissolved when the water was cooler it comes out of solution as the temperature rises. In contrast, most ionic and molecular solids are more soluble in warm water than in cold (Fig. 8.22). We make use of this characteristic in the laboratory to dissolve a substance and to grow crystals by letting a saturated solution cool slowly. However, a few solids containing ions that are extensively hydrated in water, such as lithium carbonate, are less soluble at high temperatures than at low. A small number of compounds show a mixed behavior. For example, the solubility of sodium sulfate decahydrate increases up to 32°C but then decreases as the temperature is raised further. 

Electrical conductivity can also be an important consideration in p3rrotechruc theory [7]. This phenomenon results from the presence of mobile electrons in the solid that migrate when an electrical potential is applied across the material. Metals are the best electrical conductors, while ionic and molecular solids are generally much poorer, serving well as insulators. 

Gordon, R.G. and LeSar, R. (1990) Local density functional theories of ionic and molecular solids, Adv. Quantum Chem. 21, 341-363. 

The last two pages dealt with ionic and molecular solids. Now we come to some other types of solid the metals and carbon. 

Shirley E L 1998 Many-body effects on bandwidths in ionic, noble gas, and molecular solids Phys. Rev. B 58 9579-83... 

Filtration is the separation of solids from a fluid. While various filtration processes do, in fact, include separation in the ionic and molecular ranges, here we are primarily concerned with particulate filtration. Traditionally, a filter medium is employed to remove particles in the range of 1 to 1,000 micron ( x). 

Distinguish metallic solids, ionic solids, network solids, and molecular solids by their structures and by their properties (Sections 5.8-5.11 and 5.14). 

Explain the differences among atomic, ionic, and molecular crystalline solids. 

This activity includes models of atomic, ionic, and molecular crystalline solids. List the names of the solids represented by each of these models. Make a generalization about the shape of the solid crystal and the type of solid. 

The distinction between ionic and polymeric solids is not absolute, and oxides of metals with low electropositive character (e.g., HgO) or in high oxidation states (e.g., Cr03) are better described as having polar covalent bonds. A few metals in very high oxidation states form molecular oxides (e.g., Mn207, 0s04). 

The MC and MD methods permitting the variation of the shape of the cell are best suited for the study of phase transitions in solids. These methods have been used to study phase transitions of a few solids in the last few years. Among these are monatomic solids such as rare gas solids, ionic solids, and molecular solids. There are, however, some inherent limitations in these methods. While certain transitions are readily investigated by these methods, others are more difficult. The b.c.c. to f.c.c. transformation of monatomic solids is an example of a transition that is readily observed (5, 7) (see Figs. 2 and 3). This transition has been observed as a function... 

Figure 4.5 Viscosity versus inverse temperature for glass-forming liquids, showing behavior classified as strong, typified by open tetrahedral networks, to fragile, typical of ionic and molecular liquids. Here Tg is defined by the criterion that nlT ) = 10 P. For most of the liquids, the viscosities seem to extrapolate to a common value of around 10" P at high temperatures, corresponding to a fundamental molecular vibrational frequency of around 10 sec-i. (Reprinted from J. Non-Cryst. -Solids, 73 1, Angell (1985), with kind permission from Elsevier Science—NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Dual-valency Intrinsic Semi-conductors. There is some justification for distinguishing certain compounds of the ionic and molecular ionic type, which have some of the properties associated with defect solids and yet are perfectly stoicheiometric and have insignificant structural defect. They are closely related to some of the other classes of defect solids, W are unique in their compositional and structural perfection. Their properties may be said to arise from an electrical defect ions of the same elements in two different valence states are present in the crystal in stoicheiometric proportions. 

Distinguish among and compare the characteristics of molecular, covalent, ionic, and metallic solids. Give two examples of each kind of solid. 

Recent advances in supramolecular chemistry have made available a wide range of synthetic supramolecular receptors for ionic and molecular species giving rise to a variety of interesting binding and structural motifs that occur both in solution and in the solid state. Extraction studies within the supramolecular realm continue to show strong potential for making significant contributions to both basic and applied research. At present, interest has been mainly focused on three main topics ... 

Nanoscopies supplemented with conventional techniques will allow the rational handling of the catalyst/reactive system based on its knowledge at the atoniic/molecular level. The application of nanoscopies in surface chemistry offers the possibility for determining the nanostructure of solid surfaces, surface reconstruction phenomena, to identify the structure of ionic and molecular adlayers, to study the dynamics of these adlayers in their adsorption and desorption at the submonolayer and monolayer (ML) level. Likewise, they are important tools to follow reactions at sohd surfaces in real time in different environments. The reader can get acquainted with the state of the art on these topics in Refs [5-12]. 

This behavior arises from the different natures of the components in these two solids. Common salt, NaCl, is an ionic solid that contains Na and Cl ions. When solid sodium chloride dissolves in water, sodium ions and chloride ions are distributed throughout the resulting solution. These ions are free to move through the solution to conduct an electric current. Table sugar (sucrose), on the other hand, is composed of neutral molecules that are dispersed throughout the water when the solid dissolves. No ions are present, and the resulting solution does not conduct electricity. These examples illustrate two important types of crystalline solids ionic solids, represented by sodium chloride and molecular solids, represented by sucrose. 

In last century the knowledge of defects in a solid, especially an oxide, has been explored comprehensively. The contribution of Schottky and Wagner successfully put the problem on a quantitative basis and promote the discovery of semiconductor transistor. The idea of non-stoichiometry was developed by Berthollet more than a hundred years ago and the controversy between berthollides, which do not obey the Dalton s law, and daltonides, which follow Dalton s law of constant and multiple proportions based originally upon the study of simple ionic and molecular species, encouraged the scientific debates on how existence of point defect in a compound is is it random statistic distribution or the structure related The experimental data are the best way to explore the truth. Indirect and direct observations of atom... 

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