Crystals sodium chloride

The dependence of solubility on temperature affects the mode of crystallization. For example, Figure 4 shows that the solubiUty of potassium nitrate is strongly influenced by the system temperature but that temperature has Httle influence on the solubiUty of sodium chloride. As a consequence, a reasonable yield of KNO crystals can be obtained by cooling a saturated feed solution on the other hand, cooling a saturated sodium chloride solution accomplishes Httle crystallization, and evaporation is required to increase the yield of sodium chloride crystals. 

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. 

The sodium chloride crystal contains an equal number of sodium atoms and chlorine atoms,... 

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. 

V< Animiation Figure 2.1 Sodium chloride crystal melting... 

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... 

A sodium chloride crystal contains equal numbers of Na" cations and Cl anions packed together in an alternating cubic array. Figure 2-24 illustrates a portion of the sodium chloride array. Electrical forces hold the cations and anions in place. Each Na cation attracts all the nearby Cl anions. Likewise, each Cl anion attracts all its Na neighbors. Positive cations and negative anions group together in equal numbers to make the entire collection neutral. 

For sodium cations and chloride anions, q = + and q2 — - 1. To complete the calculation, we need to know how closely the ions approach each other before their mutual attraction is balanced by electron cloud repulsion. In the sodium chloride crystal this distance is 313 pm. Using this value for r, we can calculate the energy released in... 

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.)...

FIGURE7.2 A layer ofions in the sodium chloride crystal structure. 

By means of the radius ratio, we have already described the type of local environment around the ions in several types of simple crystals. For example, in the sodium chloride structure (not restricted to NaCl itself), there are six anions surrounding each cation. The sodium chloride crystal structure is shown in Figure 7.4. 

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 ... 

This formula means that there are 0.045 too few cations present compared to those required to build the sodium chloride crystal, which must create an equivalent number of cation vacancies. 

Figure 2 In the sodium chloride crystal, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions.

In the sodium chloride crystal, each chloride ion (Cl ) is surrounded by six sodium ions (Na" ). Similarly, each Na" ion is surrounded by six Cl ions. In CsCl, each cesium ion (Cs" ) ion is surrounded hy eight Cl ions. 

Ionic Atmosphere, Before considering the distribution of ions in an ionic solution, it is instructive to consider the arrangement (distribution) of ions in an ionic crystal. For example, in a sodium chloride crystal, each ion is surrounded by six nearest neighbors of opposite charge. Each positive Na ion is surrounded by six negative Cl ions, and each negative CP ion is surrounded by six positive Na" ions (Figs. 2.9 and 2.14). 

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. 

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