Sodium chloride crystal lattice

The lattice enthalpy ofNaCl(s) is an index of the strength ofthe bonding between Na and Cl ions in the sodium chloride crystal. However, ions in a sodium chloride crystal lattice are not paired, and each sodium (or chloride) ion is attracted by six oppositely charged neighbouring ions and (less strongly) by more distant ions. This means that AH f is a measure of the total attractive force between sodium and chloride ions within the lattice, and does not simply reflect the force of attraction between isolated pairs ofNa", Cl . 

FIGURE 5.2 Structure of sodium chloride, (a) Model of the three-dimensional sodium chloride crystal lattice, (b) Each Ch ion is surrounded by 6 Na+ ions, (c) Each Na+ ion (in red) is surrounded by 6 Cl ions. 

Electric Force Cation—anion (in a crystal) Relative Strength Very strong Type CQ Example Sodium chloride crystal lattice... 

Figure 9. A RH perfusion study for crystalline sodium chloride. From 0 to 72% RH, typical water sorption exotherms are produced. Above 72% RH the water sorption no longer reaches an equilibrium, hence, the power does not return to the baseline. The sodium chloride crystal lattice begins to dissolve at 75% RH. In time all the solid will be solvated and the calorimetric signal will return to zero.

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. 

Magnesium oxide is an ionic solid that crystallizes in the sodium chloride type lattice. 

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. 

G. Tammann found that potassium and sodium chlorides form a continuous series of mixed crystals between 660° and 500°. Since neither salt has a transition point, the phenomena observed when the mixed crystals are cooled must be attributed to separation of the components. With diminishing temperature, therefore, either the attractive forces within the molecules of the respective chloride must increase, or those between the unlike molecules must be greatly weakened. The results obtained by etching the individual crystals at the ordinary temperature indicate that the intra-molecular forces of the potassium chloride crystals differ from those of the sodium chloride crystal, or, more precisely, that certain lattice regions are more closely united in the former, whilst such differences are not observed in the latter. In the light of these observations, it is surprising that the X-ray analysis indicates the same lattice for each crystal. 

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. 

Upon evaporation of (lie sulvent, the salt is obtained as such, frequently as crystals, sometimes with and sometimes without water of crystallization. A salt, when dissolved in an ionizing solvent, or fused (e.g., sodium chloride in water), is a good conductor of electricity and when rn the solid state forms a crystal lattice (e.g., sodium chloride crystals possess a definite lattice structure tor both sodium cations (Na+) and chloride anions (Cl-), determinable by examination with x-rays). 

The type of lattice formed by a particular compound does not necessarily define the character of the bond. Thus, although PbS, PbSe and PbTe crystallize with a sodium chloride type lattice which is normally associated with a purely electrostatic force between the ions, these compounds possess, in some part, the properties of a metal. ... 

Bragg (1912) showed a sodium chloride crystal to consist, not of discrete molecules of NaCl, but of Na+ ions and Cl ions arranged in an indefinitely extended cubic lattice (Fig. 28, exterior view Fig. 84 on p. 138 shows co-ordination) X-ray analysis (p. 141) gave the internuclear distance as 2.81 A. Most alkali metal halides have the same type of crystal lattice but the inter-ionic distances differ. 

These are crystalline compounds made by heating the metal in hydrogen calcium, for instance, reacts at 150°. Those of the alkali metals, XH, have the sodium chloride type lattice (p. 141) those of the Gp. IIA metals are less regular. All have stoichiometric compositions and the crystals are ionic, being somewhat denser than the metal from which they are made owing to the strong polar bonds in the ionic lattice. 

Figure 1.1 Space lattice of sodium chloride crystal. Each sodium ion is octahedrally surrounded by six chloride ions and each chloride ion is octahedrally surrounded by six sodium ions.

Coloured sodium chloride crystals are due to the formation of non-stoichiometric vacancies in the anion lattice. This vacancy is capable of trapping an electron, which can then move between a number of quantized levels. These transitions occur in the visible region and generate the yellow colour. This type of vacancy in the anion sublattice of an alkali metal halide is called a Farbenzcentre or F centre. F centres can be generated by irradiation to ionize the anion, or by exposure of the lattice to excess alkali-metal cation vapour. Both procedures result in more alkali-metal cations than halide anions in the lattice. 

This thinking in models can be relayed for example as in the perception of Max von Laue, who in 1912 confirmed the structural theory of 3-dimensional crystal lattices by using a beam of X-rays [4], The interference pattern of a sodium chloride crystal, which through interference and diffraction of the X-ray-beam is formed, is the original, and therefore the essential part, passing through the sieve (see Figs. 4.2 and 4.3). 

In a sodium chloride crystal, the Na" and Cl ions are arranged in a giant lattice structure. The building brick of this structure is a unit cell is as shown in Fig 4.2. The larger spheres represent Cl ions, whereas the smaller spheres represent Na" ions. A crystal of sodium chloride consists of many billions of these unit cells stacked together in the lattice. 

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