Sodium chloride unit cell

Most salts crystallize as ionic solids with ions occupying the unit cell. Sodium chloride (Figure 13-28) is an example. Many other salts crystallize in the sodium chloride (face-centered cubic) arrangement. Examples are the halides of Li+, K+, and Rb+, and M2+X2 oxides and sulfides such as MgO, CaO, CaS, and MnO. Two other common ionic structures are those of cesium chloride, CsCl (simple cubic lattice), and zincblende, ZnS (face-centered cubic lattice), shown in Figure 13-29. Salts that are isomorphous with the CsCl structure include CsBr, Csl, NH4CI, TlCl, TlBr, and TIL The sulfides of Be2+, Cd2+, and Hg2+, together with CuBr, Cul, Agl, and ZnO, are isomorphous with the zincblende structure (Figure 13-29c). 

The rocksalt stmcture is illustrated in figure Al.3.5. This stmcture represents one of the simplest compound stmctures. Numerous ionic crystals fonn in the rocksalt stmcture, such as sodium chloride (NaCl). The conventional unit cell of the rocksalt stmcture is cubic. There are eight atoms in the conventional cell. For the primitive unit cell, the lattice vectors are the same as FCC. The basis consists of two atoms one at the origin and one displaced by one-half the body diagonal of the conventional cell. 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

In compound materials - in the ceramic sodium chloride, for instance - there are two (sometimes more) species of atoms, packed together. The crystal structures of such compounds can still be simple. Figure 5.8(a) shows that the ceramics NaCl, KCl and MgO, for example, also form a cubic structure. Naturally, when two species of atoms are not in the ratio 1 1, as in compounds like the nuclear fuel UO2 (a ceramic too) the structure is more complicated (it is shown in Fig. 5.8(b)), although this, too, has a cubic unit cell. 

Potassium iodide has a unit cell similar to that of sodium chloride (Figure 9.18). The ionic radii of K+ and I- are 0.133 nm and 0.216 nm, respectively. How long is... 

Consider the sodium chloride unit cell shown in Figure 9.18. Looking only at the front face (five large Cl ions, four small Na+ ions),... 

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... 

For sodium chloride, since there are eight molecules per unit cell, and the cell volume is ao where a0 is the lattice parameter, the energy per unit volume becomes ... 

Potassium fluoride crystallizes in a sodium chloride lattice. The length of the edge of the unit cell (sometimes called the cell or lattice constant) has the value 267 pm for KF. 

The method can be illustrated by reference to a classical 1933 study of the defects present in wilstite, iron monoxide. Wustite adopts the sodium chloride (NaCl) structure, and the unit cell should contain 4 Fe and 4 O atoms in the unit cell, with an ideal composition FeOi.o, but in reality the composition is oxygen rich and the unit cell dimensions also vary with composition (Table 1.1). Because there is more oxygen present than iron, the real composition can be obtained by assuming either that there are extra oxygen atoms in the unit cell (interstitial defects) to give a composition FcO 1 +v, or that there are iron vacancies present, to give a formula Fci-JD. It is... 

Assume that the iron atoms in the crystal are in a perfect array, identical to the metal atoms in the sodium chloride structure, and that the 0.058 excess of oxygen is due to interstitial oxygen atoms being present, over and above those on the normal anion positions. The unit cell of the structure now contains 4 Fe and (4 x 1.058) O. The density is calculated to be 6076 kg m-3. 

Assume that the oxygen array is perfect and identical to the nonmetal atom array in the sodium chloride structure and that the unit cell contains some vacancies on the iron positions. In this case, one unit cell will contain 4 atoms of oxygen and (4/1.058) atoms of iron, that is, 4Feo.94sO. The density is calculated to 5741 kg m-3. 

Figure 1.14 Crystal structure of sodium chloride, NaCl (a) a perspective view of one unit cell and (h) projection down [010], the b axis.

Pure potassium bromide, KBr, which adopts the sodium chloride structure, has the fraction of empty cation sites due to Schottky defects, ncv/Nc, equal to 9.159xl0-21 at 20°C. (a) Estimate the enthalpy of formation of a Schottky defect, Ahs. (b) Calculate the number of anion vacancies per cubic meter of KBr at 730°C (just below the melting point of KBr). The unit cell of KBr is cubic with edge length a = 0.6600 nm and contains four formula units of KBr. 

The formation energy of Schottky defects in NiO has been estimated at 198 kJ mol-1. The lattice parameter of the sodium chloride structure unit cell is 0.417 nm. (a) Calculate the number of Schottky defects per cubic meter in NiO at 1000°C. (b) How many vacancies are there at this temperature (c) Estimate the density of NiO and hence the number of Schottky defects per gram of NiO. 

We now introduce a Fourier transform procedure analogous to that employed in the solution theory, s 62 For the purposes of the present section a more detailed specification of defect positions than that so far employed must be introduced. Thus, defects i and j are in unit cells l and m respectively, the origins of the unit cells being specified by vectors R and Rm relative to the origin of the space lattice. The vectors from the origin of the unit cell to the defects i and j, which occupy positions number x and y within the cell, will be denoted X 0 and X for example, the sodium chloride lattice is built from a unit cell containing one cation site (0, 0, 0) and one anion site (a/2, 0, 0), and the translation group is that of the face-centred-cubic lattice. However, if we wish to specify the interstitial sites of the lattice, e.g. for a discussion of Frenkel disorder, then we must add two interstitial sites to the basis at (a/4, a/4, a]4) and (3a/4, a/4, a/4). (Note that there are twice as many interstitial sites as anion-cation pairs but that all interstitial sites have an identical environment.) In our present notation the distance between defects i and j is... 

The chlorine liquefaction plant comprises a bromine-removal column, a compression-condensation unit and a Tetra absorption/distillation unit (Fig. 14.2). Waste streams of chlorine are absorbed in diluted cell-liquor in the chlorine destruction area. As a result, the destruction liquid contains sodium chloride and less sodium hydroxide than is usual. Bromine from the bromine-removal column is also added to the chlorine destruction unit. The hypochlorite solution that is formed contains a reasonable amount of bromine and salts. However, it is a hypochlorite of non-marketable quality. 

In general, any ionic solid can be pictured as a unit cell in which the cations are surrounded by a number of anions and the anions are surrounded by a number of cations. A typical example is that of sodium chloride (Fig. 6.1). 

The sodium chloride structure, AX systems. Cubic Fm3m (Space Group 225) The sodium chloride or rock salt, NaCl, structure has a simple face-centered cubic unit cell (Figure 8) with alternating cations-anions along the three cubic axes. 

An alternative way of viewing this structure is to think of it as a cubic close-packed array of chloride Ions with sodium Ions filling all the octahedral holes. The conventional unit cell of a ccp array Is an F face-centred cube (hence the cubic in ccp) the close-packed layers lie at right angles to a cube diagonal (Figure 1.32). Filling all the... 

The sodium chloride unit cell contains four formula units of NaCl. If you find this difficult to see, work it out for yourself by counting the numbers of ions in the different sites and applying the information given in Table 1.4. 

If you have a class with biochemists, clearly the area of enzyme kinetics is practically mandatory. If biologists are mixed in with the biochemists, osmotic pressure is an important concept to cover carefully and a concept typically not well covered in general chemistry and in most physical chemistry texts or classes. A quick example what is a 2 Osmolar solution of sodium chloride Such concentration units are used when dispensing various saline solutions in hospitals. What is the origin of the unit A 1 M NaCl solution dissociates into two ions that would double the osmotic pressure of a non dissociating solute. Thus, the 1 M solution of NaCl becomes a 2 Osmolar solution. Other examples abound - the bursting pressure of a cell relates to the osmotic pressure of the serum in which the cell finds itself. 

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