Sodium chloride crystallization with

Figure 4-10 gives intensity distributions for crystals used in x-ray emission spectrography during 1958 in the authors laboratory. Each of the patterns shows some broadening. Only in the case of the sodium chloride crystal with the major flaw was the broadening serious enough to produce interference with the Ka lines of adjacent elements. 

An ionic compound typically contains a multitude of ions grouped together in a highly ordered three-dimensional array. In sodium chloride, for example, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions (Figure 6.11). Overall there is one sodium ion for each chloride ion, but there are no identifiable sodium-chloride pairs. Such an orderly array of ions is known as an ionic crystal. On the atomic level, the crystalline structure of sodium chloride is cubic, which is why macroscopic crystals of table salt are also cubic. Smash a large cubic sodium chloride crystal with a hammer, and what do you get Smaller cubic sodium chloride crystals Similarly, the crystalline structures of other ionic compounds, such as calcium fluoride and aluminum oxide, are a consequence of how the ions pack together. 

In order to test the effect of the shape of crystals, a sodium chloride crystal with (111) and (-1-1-1) faces, together with 100 faces, has been examined. In the crystal 28 cations and 28 anions are included, and the (111) face consists of only sodium ions, and the (-1-1-1) face of chloride ions. No significant difference has in fact been found in the dissolution mechanism of crystals with different faces. 

Microscopically the distinction can be observed in the definite shapes of crystals, which reflect the regular atomic arrangements compare, for example, the cubic faces of common salt (sodium chloride) crystals with the irregular and often... 

In the sodium chloride crystal, the Na+ ion is slightly too large to fit into holes in a face-centered lattice of Cl- ions (Figure 9.18). As a result, the Cl- ions are pushed slightly apart so that they are no longer touching, and only Na+ ions are in contact with Cl- ions. However, the relative positions of positive and negative ions remain the same as in LiCk Each anion is surrounded by six cations and each cation by six anions. 

FIGURE 14.17 A diaphragm cell tor the electrolytic production of sodium hydroxide from brine (aqueous sodium chloride solution), represented by the blue color. The diaphragm (gold color) prevents the chlorine produced at the titanium anodes from mixing with the hydrogen and the sodium hydroxide formed at the steel cathodes. The liquid (cell liquor) is drawn off and the water is partly evaporated. The unconverted sodium chloride crystallizes, leaving the sodium hydroxide dissolved in the cell liquor. 

In deriving theoretical values for inter-ionic distances in ionic crystals the sum of the univalent crystal radii for the two ions should be taken, and corrected by means of Equation 13, with z given a value dependent on the ratio of the Coulomb energy of the crystal to that of a univalent sodium chloride type crystal. Thus, for fluorite the sum of the univalent crystal radii of calcium ion and fluoride ion would be used, corrected by Equation 13 with z placed equal to y/2, for the Coulomb energy of the fluorite crystal (per ion) is just twice that of the univalent sodium chloride structure. This procedure leads to the result 1.34 A. (the experimental distance is 1.36 A.). However, usually it is permissible to use the sodium chloride crystal radius for each ion, that is, to put z = 2 for the calcium... 

In addition to crystal size and size distribution, the shape of the crystal product might also be important. The term crystal habit is used to describe the development of faces of the crystal. For example, sodium chloride crystallizes from aqueous solution with cubic faces. On the other hand, if sodium chloride is crystallized from an aqueous solution... 

Figure 2.2 A contour plot of the electron density in a plane through the sodium chloride crystal. The contours are in units of 10 6 e pm-3. Pauling shows the radius of the Na+ ion from Table 2.3. Shannon shows the radius of the Na+ ion from Table 2.5. The radius of the Na+ ion given by the position of minimum density is 117 pm. The internuclear distance is 281 pm. (Modified with permission from G. Schoknecht, Z Naiurforsch 12A, 983, 1957 and J. E. Huheey, E. A. Keiter, and R. L. Keiter, Inorganic Chemistry, 4th ed., 1993, HarperCollins, New York.)...

Lead is found as the sulfide, but the other members of the group also form compounds with sulfur. Although PbS has the sodium chloride crystal structure, a silicon sulfide having the formula SiS2 is known that has a chain structure ... 

Spectra of the compounds with sodium chloride crystal structure (Fig. 24) show strong resemblance. Quantitative correlation between lattice parameters and absorption maxima is poor as seen on Table II. 

Magnesium oxide crystals about 500 A. in diameter were prepared in vacuo by Nicolson 26). Lattice determinations by X-rays showed that the parameter of these small crystals was smaller than that of large crystals. The surface tension obtained from these experiments (- -3,020 dynes/cm.) was 46% of the theoretical value. Similar experiments were carried out with sodium chloride crystals made in vacuo (size about 2000 A), and the agreement between experiment and theory was better, the observed surface tension (- -390 dynes/cm.) being 70% of that calculated. 

Also the effect of impurities in a crystal on the Vickers hardness was analysed. In Figure 4 are shown the force dependency curves of the Vickers hardnesses of a pure sodium chloride crystal and a sodium chloride crystal grown in a solution with an impurity of 10 % urea. The hardness of the pure sodium chloride crystal is up to 25 % higher than the hardness of the impure crystal. 

Figure 4. Force-dependency of the Vickers hardness for pure sodium chloride and sodium chloride crystals grown in a solution with an impurity of 10 % urea...

G. Tammann found that potassium and sodium chlorides form a continuous series of mixed crystals between 660° and 500°. Since neither salt has a transition point, the phenomena observed when the mixed crystals are cooled must be attributed to separation of the components. With diminishing temperature, therefore, either the attractive forces within the molecules of the respective chloride must increase, or those between the unlike molecules must be greatly weakened. The results obtained by etching the individual crystals at the ordinary temperature indicate that the intra-molecular forces of the potassium chloride crystals differ from those of the sodium chloride crystal, or, more precisely, that certain lattice regions are more closely united in the former, whilst such differences are not observed in the latter. In the light of these observations, it is surprising that the X-ray analysis indicates the same lattice for each crystal. 

There is one other element of symmetry possessed by sodium chloride crystals. For each face, edge, or corner of the cube or octahedron there is an exactly similar face, edge, or comer diametrically opposite the centre of the cube or octahedron (Fig. 19) is therefore called a centre of symmetry. The centre of symmetry possessed by these shapes corresponds with the centre of symmetry in the atomic arrangement the centre of any sodium or chlorine ion is a centre of symmetry, since along any direction from the selected ion the arrangement encountered is exactly repeated in the diametrically opposite direction. 

Since the electron distribution function for an ion extends indefi-finitely, it is evident that no single characteristic size can be assigned to it. Instead, the apparent ionic radius will depend upon the physical property under discussion and will differ for different properties. We are interested in ionic radii such that the sum of two radii (with certain corrections when necessary) is equal to the equilibrium distance between the corresponding ions in contact in a crystal. It will be shown later that the equilibrium interionic distance for two ions is determined not only by the nature of the electron distributions for the ions, as shown in Figure 13-1, but also by the structure of the crystal and the ratio of radii of cation and anion. We take as our standard crystals those with the sodium chloride arrangement, with the ratio of radii of cation and anion about 0.75 and with the amount of ionic character of the bonds about the same as in the alkali halogenides, and calculate crystal radii of ions such that the sum of two radii gives the equilibrium interionic distance in a standard crystal. 

Upon evaporation of (lie sulvent, the salt is obtained as such, frequently as crystals, sometimes with and sometimes without water of crystallization. A salt, when dissolved in an ionizing solvent, or fused (e.g., sodium chloride in water), is a good conductor of electricity and when rn the solid state forms a crystal lattice (e.g., sodium chloride crystals possess a definite lattice structure tor both sodium cations (Na+) and chloride anions (Cl-), determinable by examination with x-rays). 

In this cell, not all of the dissolved sodium chloride is electrolyzed. Consequently, the solution that seeps through the perforated cathode contains sodium hydroxide together with some unchanged sodium chloride. The solution is concentrated by evaporation, whereupon most of the less soluble sodium chloride crystallizes and the very soluble sodium hydroxide remains in solution. This concentrated solution of sodium hydroxide (caustic soda) may be sold as such, or the remainder of the water may be driven off by heating to form solid sodium hydroxide. If a purer product is desired, the solid is dissolved in alcohol, which does not dissolve the remaining traces of sodium chloride. Pure sodium hydroxide is then secured by filtration, followed by evaporation of the alcohol. 

The ionic hydrides are white solids with high melting points, and all of the alkali metal hydrides have the sodium chloride crystal structure. Because they resemble the salts of the alkali and alkaline earth metals, the ionic hydrides are often referred to as saline or salt-like hydrides. The properties of the alkali metal hydrides are shown in Table 6.3, and those of the alkaline earth hydrides are shown in Table 6.4. 

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