Sodium chloride coordination number

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

In a sodium chloride type of lattice, there are six cations around each anion and six anions around each cation. These oppositely charged ions are arranged as follows one above, one below, one in front, one in back, one to the right, and one to the left. Thus the coordination number of Mg2+ is 6 and that of O2" is 6 also. 

In simple binary structures, in which no other relevant coordinations (or bonds) can be considered, the ratio m/n will be equal to the stoichiometric compositional ratio. For instance, we will write NaCl6/6 to represent the hexa-coordination (in this case octahedral coordination) of Cl around Na (and vice versa) in sodium chloride. Similarly we will have ZnS4/4 PH3/1 CsCl8/8 CaF8/4 UCI9/3 etc. Moreover, it is possible to add modifiers to the coordination numbers in order to specify not only topological but also geometrical characteristics of the primary coordination sphere. 

The sodium chloride structure. Sodium chloride crystallizes in a face-centered cubic structure (Fig. 4.1a). To visualize the face-centered arrangement, consider only the sodium ions or the chloride ions (this will require extensions of the sketch of the lattice). Eight sodium ions form the comers of a cube and six more are centered on the faces of the cube. The chloride ions are similarly arranged, so that the sodium chloride lattice consists of two interpenetrating face-centered cubic lattices. The coordination number (C.N.) of both ions in the sodium chloride lattice is 6. that is, there are six chloride ions about each sodium ion and six sodium ions about each chloride ion. 

In the same way we can predict that sodium ions will prefer octahedral holes m a closest packed lattice of chloride ions (rNB Acr 116 pm/167 pm = 0.69), forming the well-known sodium chloride lattice with a coordination number of 6 (Fig. 4.1a). 

Alternatively, we might examine the radius ratio of Oj BF and get a crude estimate of = 0.8. The accuracy of our values does not permit us to choose between coordination number 6 and 8, but since the value of the Madelung constant does not differ appreciably between the sodium chloride and cesium chloride structures, a value of 1.75 may be taken which will suffice for our present rough calculations. We may then use the Bom-Lande equation (Eq. 4.13), which provides an estimate of —616 kJ mor1 for the attractive energy, which will be decreased by about 10% (if... 

Consider now the sodium chloride crystal structure shown in Figure 9-40. It is built from sodium ions and chloride ions, and it is kept together by electrostatic forces. The chloride ions are much larger than the sodium ions. As equal numbers of cations and anions build up this structure, the maximum number of neighbors will be the number of the larger chloride ions that can be accommodated around the smaller sodium ion. The opposite would not work although more sodium ions could surround a chloride ion, the same coordination... 

Pfeiffer regarded sodium chloride as a highmolecu-lar-weight coordination compound (NaCl) constructed of equal numbers of [NaCle] and [ClNa ] units. The difference between primary valences and secondary valences disappears in crystals of symmetrical compounds. He demonstrated that, in other crystals, coordination centers can be groups of atoms as well as single atoms, and that coordination numbers as high as 12 must sometimes be considered. He proposed that, in crystals of simple organic molecular compounds of type AB, each constituent acts as a coordination center ABe and BA6 units interpenetrate exactly as in sodium chloride. 

Table 2. Estimated structural parameters for sodium and chloride ions and for the first neighbor water molecules in 2 mol dm" aqueous solution of sodium chloride at room temperature and 1 to 2000 bar pressure, as derived from X-ray diffraction measurements average distances ry and coordination numbers ny.

AgF, AgCl and AgBr crystallize with a sodium chloride lattice and CuCl, CuBr, and Cul with the lattice given in Figure 62 in which copper has a coordination number of four. This difference in crystal structure of the two salts is not determined by the difference in the dimensions of the Ag"+ and Cu+ ions, since according to Pauling the latter has the value 0-96 A, which is not outside the limits for a sodium chloride lattice. The decrease of the coordination number from six to four and the formation of a tetrahedral configuration are more probably determined by the covalent character of the bonds between copper and chlorine. However, it is to be emphasized that the bonds are not entirely covalent any more than they are entirely ionic, but are of an intermediate... 

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