Crystals of sodium chloride

The magnificent purple pigment referred to in the Bible and known to the Romans as Tyrian purple after the Phoenician port of Tyre (Lebanon), was shown by P. Friedlander in 1909 to be 6,6 -dibromoindigo. This precious dye was extracted in the early days from the small purple snail Murex brandaris, as many as 12000 snails being required to prepare 1.5 g of dye. The element itself was isolated by A.-J. Balard in 1826 from the mother liquors remaining after the crystallization of sodium chloride and sulfate from the waters of the Montpellier salt marshes ... 

We have, in this chapter, encountered a number of properties of solids. In Table 5-II, we found that melting points and heats of melting of different solids vary widely. To melt a mole of solid neon requires only 80 calories of heat, whereas a mole of solid copper requires over 3000 calories. Some solids dissolve in water to form conducting solutions (as does sodium chloride), others dissolve in water but no conductivity results (as with sugar). Some solids dissolve in ethyl alcohol but not in water (iodine, for example). Solids also range in appearance. There is little resemblance between a transparent piece of glass and a lustrous piece of aluminum foil, nor between a lump of coal and a clear crystal of sodium chloride. 

FIGURE C.3 An ionic solid consists of an array of cations and anions stacked together. This illustration shows the arrangement of sodium cations (Na+) and chlorine anions (chloride ions, Cl-) in a crystal of sodium chloride (common table salt). The faces of the crystal are where the stacks of ions come to an end. 

The formulas of ionic compounds have a different meaning from those of molecular compounds. Each crystal of sodium chloride has a different total number of cations and anions. We cannot simply specify the numbers of ions present as the formula of this ionic compound, because each crystal would have a different formula and the subscripts would be enormous numbers. However, the ratio of the number of cations to the number of anions is the same in all the crystals, and the chemical formula shows this ratio. In sodium chloride, there is one Na+ ion for each Cl ion so its formula is NaCl. Sodium chloride is an example of a binary ionic compound, a compound formed from the ions of two elements. Another binary compound, CaCl2, is formed from Ca2+ and Cl- ions in the ratio 1 2, which is required for electrical neutrality. 

To understand why a crystal of sodium chloride, an ionic compound, has a lower energy than widely separated sodium and chlorine atoms, we picture the formation of the solid as taking place in three steps sodium atoms release electrons, these electrons attach to chlorine atoms, and then the resulting cations and anions clump together as a crystal. Chemists often analyze complex processes by breaking them down into simpler steps such as these, and often consider hypothetical steps (steps that do not actually occur). 

The rock-salt structure is a common ionic structure that takes its name from the mineral form of sodium chloride. In it, the Cl- ions lie at the corners and in the centers of the faces of a cube, forming a face-centered cube (Fig. 5.39). This arrangement is like an expanded ccp arrangement the expansion keeps the anions out of contact with one another, thereby reducing their repulsion, and opens up holes that are big enough to accommodate the Na+ ions. These ions fit into the octahedral holes between the Cl ions. There is one octahedral hole for each anion in the close-packed array, and so all the octahedral holes are occupied. If we look carefully at the structure, we can see that each cation is surrounded by six anions and each anion is surrounded by six cations. The pattern repeats over and over, with each ion surrounded by six other ions of the opposite charge (Fig. 5.40). A crystal of sodium chloride is a three-dimensional array of a vast number of these little cubes. 

FIGURE 5.40 Bill ions of unit cells stack together to recreate the smooth faces of the crystal of sodium chloride seen in this micrograph. The first inset shows some of... 

In an x-ray diffraction experiment on a single crystal of sodium chloride, with the use of radiation from a copper source (X = 154 pm), constructive interference was observed at 0 = 11.2°. What is the spacing of the layers responsible for the diffraction ... 

Figure I. A crystal of sodium chloride (lower left) showing arrangement of atoms within the crystal and two principal directions of growth. Growth perpendicular to cube face encounters layers of atoms shown at upper left. Growth along diagonal of cube encounters alternating layers shown at right...

Alberger A process for crystallizing sodium chloride from brine. The brine is heated under pressure to 145°C to remove calcium sulfate. Flashing to atmospheric pressure produces line cubic crystals of sodium chloride, and surface evaporation in circular vessels produces flakes of it. Developed by J. L. and L. R. Alberger in the 1880s. See also Reciystallizer. 

Many substances do not exist as molecules. For example, the atoms in most inorganic solids are in a three-dimensional structure in which each atom is surrounded by a number of other atoms. In crystals of sodium chloride, no distinct Na and Cl pair can be called a molecule, because each sodium is surrounded by 6 chlorine and each chlorine is surrounded by 6 sodium. 

Cone. soln. of sodium hypochlorite with up to 42 per cent, of available chlorine have been made under the trade name chloros, by passing chlorine into a soln. of caustic soda of such a strength that the sodium chloride which is formed separates out. The temp, is kept below 27°. The crystals of sodium chloride are removed, and more chlorine is introduced, but the sodium hydroxide is always kept in excess or the soln. will be unstable. A. J. Balard prepared potassium, sodium, and lithium hypochlorites by neutralizing a well-cooled soln. of the base with the acid. E. Soubeiian evaporated in vacuo the liquid obtained by treating a soln. of calcium hypochlorite with sodium carbonate, and obtained, before the liquid had all evaporated, crystals of sodium chloride and of sodium hypochlorite. P. Mayer and R. Schindler obtained solid potassium hypochlorite mixed with potassium hydrocarbonate by the action of chlorine—developed from 10 parts of sodium chloride—on a soln. of 24 parts of potassium hydrocarbonate and one of water. 

Above layer formation on crystal of cadmium iodide ( X 600). Below, left layer formation on crystal of sodium chloride ( x 1400). Below, right skeletal growths of ammonium... 

The charges in sodium chloride are balanced, but they are not neutralized. As a water molecule gets close to the sodium chloride, it can distinguish the various ions and it is thus attracted to an individual ion by ion—dipole forces. This works because sodium and chloride ions and water molecules are of the same scale. We, on the other hand, are much too big to be able to distinguish individual ions within a crystal of sodium chloride. From our point of view, the individual charges are not apparent. 

Nitric Acid. — Shake 1 gm. of molybdic anhydride with 10 cc. of water, and add a small crystal of sodium chloride, followed by one drop of a 1 1000 solution of indigo the blue color of the solution must not disappear on adding 10 cc. of concentrated sulphuric acid. 

A single crystal of sodium chloride for an X-ray structure determination is a cube 0.3 mm on a side. 

FIGURE 2.10 The arrange- ment of Na+ ions and Cl- ions in a crystal of sodium chloride. Each Na+ ion is surrounded by six neighboring Cl- ions, and each Cl- ion is surrounded by six neighboring Na+ ions. 

The oppositely charged Na+ and Cl- ions that result when sodium transfers an electron to chlorine are attracted to one another by electrostatic forces, and we say that they are joined by an ionic bond. The crystalline substance that results is said to be an ionic solid. A visible crystal of sodium chloride does not consist of individual pairs of Na+ and Cl- ions, however. Instead, solid NaCl consists of a vast three-dimensional network of ions in which each Na+ is surrounded by and attracted to many Cl - ions, and each Cl- is surrounded by and attracted to many Na+ ions (Figure 6.7). 

The latter formula has been established in a manner similar to the former. A. Werner assumes that four of the chlorine atoms are attached to platinum by principal valencies, and two by auxiliary valencies and that the last two chlorine atoms are not saturated so that the hydrogen atoms may attach themselves at these points. T. M. Lowry modified the idea by assuming that the quadrivalent platinum atom begins by attracting to itself a shell of six chlorine ions as in la of the following schemes where the electrostatic attractions or electrovalencies are represented by dotted lines. The orientation of these six ions is identical with that of the six chlorine ions distributed about each ion of sodium in a crystal of sodium chloride-16. The outer shell... 

When X-rays are passed through a crystal of sodium chloride, for example, you get a pattern of spots called a diffraction pattern (Figure 3.15b). This pattern can be recorded on photographic film and used to work out how the ions or atoms are arranged in the crystal. Crystals give particular diffraction patterns depending on their structure, and this makes X-ray diffraction a particularly powerful technique in the investigation of crystal structures. 

Figure 3.l5d shows only a tiny part of a small crystal of sodium chloride. Many millions of sodium ions and chloride ions would be arranged in this way in a crystal of sodium chloride to make up the giant ionic structure. Each sodium ion in the lattice is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. 

Ross and Boyd (II) prepared crystals of sodium chloride in which both the 100 and the HI surfaces were developed the adsorption isotherm of ethane at 90.1° K gave evidence of two type 2—i.e., near-homotattic—surfaces, as well as showing the initial knee, which is evidence of a type 1 surface. This isotherm is shown in Figure 4, which also includes for comparison an ethane isotherm measured at the same temperature for a sodium chloride adsorbent that had only 100 faces developed the pressure characteristic of the phase transition of ethane on the homotattic 190 surface of sodium chloride at 90.1° K shows on both isotherms as the location of a discontinuity at p = 4.5 X 10-3 mm. the homotattic ill portion of the surface is responsible for the convex shape (with respect to the pressure axis) of the isotherm beyond the knee, though the rise is not suffi-... 

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