Sodium chloride crystalline structure

This indicates that, in crystalline sodium chloride (for structure, click here http //bit.lv/gXzbqk or see Appendix 2), each sodium atom (ion) is surroimded by six chlorine atoms (ions), each of which is shared between six sodium atoms (ions), giving an empirical formula NaCl. Crystallographers call the munber of nearest neighbours the coordination number . 

Feitknecht has examined the corrosion products of zinc in sodium chloride solutions in detail. The compound on the inactive areas was found to be mainly zinc oxide. When the concentration of sodium chloride was greater than 0-1 M, basic zinc chlorides were found on the corroded parts. At lower concentrations a loose powdery form of a crystalline zinc hydroxide appeared. A close examination of the corroded areas revealed craters which appeared to contain alternate layers and concentric rings of basic chlorides and hydroxides. Two basic zinc chlorides were identified, namely 6Zn(OH)2 -ZnClj and 4Zn(OH)2 ZnCl. These basic salts, and the crystalline zinc hydroxides, were found to have layer structures similar in general to the layer structure attributed to the basic zinc carbonate which forms dense adherent films and appears to play such an important role in the corrosion resistance of zinc against the atmosphere. The presence of different reaction products in the actual corroded areas leads to the view that, in addition to action between the major anodic and cathodic areas as a whole, there is also a local interaction between smaller anodic and cathodic elements. 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

Figure 1.7 The structures of crystalline sodium chloride (NaCI), cesium chloride (CsCl), and zinc sulfide (ZnS).

The structure in Figure 2.14 shows the result of an ionic reaction sodium metal has reacted with chlorine gas to yield white crystalline sodium chloride, NaCl. Each Na atom has lost an electron to form an Na+ cation and each chlorine atom has gained an electron and is hence a CP anion. In practice, the new electron possessed by the chloride came from the sodium atom. 

Solid sodium chloride has a crystalline structure in which the cations and anions alternate in a repeating pattern. 

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. 

An ionically bonded molecule (NaCl). (a) A sodium atom (Na) can donate the one electron in its valence shell to a chlorine atom (Cl), which has seven electrons in its outermost shell. The resulting ions (Na+ and CP) bond to form the compound sodium chloride (NaCl). The octet rule has been satisfied, (b) The ions that constitute NaCl form a regular crystalline structure in the solid state. 

Use your research skills on the Internet to find out about the technique of X-ray crystallography and how this technique can be used to determine the crystalline structure of solid substances such as sodium chloride. 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

At an earlier point (p. 185) the unit cell of a crystalline structure was described as one of a large number of identical prisms, which, when oriented in the same way and stacked together in three dimensions, form a perfect crystal. The corners of an array of unit cells put together in this way are said to be the points of a space lattice the surroundings about each point of the lattice must be identical to the surroundings about every other point. Additional lattice points may sometimes be put at the face centers or at the body centers of the unit cells in crystalline sodium chloride (Fig. 12-2), for example, chloride ions are located both at the corners and the face centers of the unit cell, and an observer at a corner would have the same surroundings as one at a face center. The description of the structure of a crystalline solid is then a description of the size and shape of the unit cell and of the locations of the atoms within it. 

One peculiarity of salt (and other substances with atoms locked in a crystalline structure by ionic bonds) is that individual molecules of sodium chloride do not exist at room temperature. What does exist is a lattice of oppositely charged ions, a crystal held together by the strong electrostatic attraction between ions. 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

Effect of Impurities on CaS04 Transformation. The transition from gypsum to orthorhombic anhydrite is slow but occurs even at ambient temperatures (44). The relatively large concentration of finely divided sodium chloride present in the plaster in the tomb of Nefertari may have facilitated the dehydration process. The presence of a hygroscopic material, such as sodium chloride, can help promote dehydration reactions. Also, impurities within the lattice of a crystalline structure can weaken the lattice (46, 47) and thereby accelerate thermodynamically favored reactions. These points suggest a strong correlation between the extent to which the dehydration reaction proceeds and sodium chloride concentration, but they do not exclude the possibility that dehydration can take place in the absence of salt. 

Solids are classified as crystalline solids or amorphous solids. Crystalline solids, such as an ice cube or a sodium chloride crystal, have a definite melting point. Amorphous solids, such as a chocolate bar or glass, get softer and softer as the temperature is raised. The structures of crystalline solids feature regularly repeating arrangements of the constituent particles. The structure of amorphous solids is not regular, but something like that of liquids sometimes, amorphous solids are called supercooled liquids. ... 

From the data on the structure of crystals discussed in this chapter we may make the same conclusion as that reached for gaseous molecules, namely, that the homopolar and the ionic bond are only limiting cases and that actual bonds, as characterized by the distribution of the electron cloud may be descrilDcd in terms of the relative contributions of the limiting structures. In the crystalline state diamond and sodium chloride may be taken as characteristic of the limiting structures. A satisfactory theory of crystals, intermediate between these extremes, will only be attained when, by wave mechanical methods, it is found possible to describe the motion of electrons in the periodic field due to the atoms arranged in the crystal. 

Some crystals are ionic, that is, long networks of ions held together by ionic attractions. Sodium chloride, table salt, is an ionic solid. In crystals of sodium chloride, a sodium ion is surrounded by six chlorine ions— one on top, one on the bottom, and one at each compass point—and each chloride ion is surrounded by six sodium ions in the same manner. Crystals of sugar, on the other hand, are held together by intermolecular attractions. They fit together for optimum balance of attraction and repulsion so that a quite orderly crystalline structure results. Pure metals are stacks of identical atoms so the bonding between one pair cannot be any different than the bonding between the next pair. Consequently, it is impossible to... 

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