Sodium chloride electron density

A better idea of the real size of an ion in a molecule can now be obtained from a study of electron density distributions, which it has recently become possible to obtain from accurate X-ray crystallographic studies of crystals. Figure 2.2 shows a contour map of the electron density distribution obtained in an X-ray crystallographic study of crystalline sodium chloride. The position of minimum electron density between two adjacent ions seems to be... 

Figure 2.2 A contour plot of the electron density in a plane through the sodium chloride crystal. The contours are in units of 10 6 e pm-3. Pauling shows the radius of the Na+ ion from Table 2.3. Shannon shows the radius of the Na+ ion from Table 2.5. The radius of the Na+ ion given by the position of minimum density is 117 pm. The internuclear distance is 281 pm. (Modified with permission from G. Schoknecht, Z Naiurforsch 12A, 983, 1957 and J. E. Huheey, E. A. Keiter, and R. L. Keiter, Inorganic Chemistry, 4th ed., 1993, HarperCollins, New York.)...

Based on the ionic radii, nine of the alkali halides should not have the sodium chloride structure. However, only three, CsCl, CsBr, and Csl, do not have the sodium chloride structure. This means that the hard sphere approach to ionic arrangement is inadequate. It should be mentioned that it does predict the correct arrangement of ions in the majority of cases. It is a guide, not an infallible rule. One of the factors that is not included is related to the fact that the electron clouds of ions have some ability to be deformed. This electronic polarizability leads to additional forces of the types that were discussed in the previous chapter. Distorting the electron cloud of an anion leads to part of its electron density being drawn toward the cations surrounding it. In essence, there is some sharing of electron density as a result. Thus the bond has become partially covalent. 

The difference in the reactivity of benzylic versus aromatic halogens makes it possible to reduce the former ones preferentially. Lithium aluminum hydride replaced only the benzylic bromine by hydrogen in 2-bromomethyl-3-chloro-naphthalene (yield 75%) [540]. Sodium borohydride in diglyme reduces, as a rule, benzylic halides but not aromatic halides (except for some iodo derivatives) [505, 541]. Lithium aluminum hydride hydrogenolyzes benzyl halides and aryl bromides and iodides. Aryl chlorides and especially fluorides are quite resistant [540,542], However, in polyfluorinated aromatics, because of the very low electron density of the ring, even fluorine was replaced by hydrogen using lithium aluminum hydride [543]. 

Close-packed spheres occupy 74.04% of a total volume, hence the hard-sphere radius of I" in these 2 1 salts in 2.03 A. Correction for the electrostatic attraction alone would give a monovalent iodide radius of about 2.24, an opposite repulsion-correction for the different co-ordination number would reduce this to about 2.10 A for the monovalent sodium-chloride type (see Appendix). Such values are consistent with our earlier estimates, but incompatible with the electron-density minimum value (4) of 1.94 A. 

Scientists, using X-ray diffraction (Figure 3.15a), have obtained photographs that indicate the way in which the ions are arranged (Figure 3.15b). The electron density map of sodium chloride is shown in Figure 3.15c. 

Electron distributions in sodium chloride — the prototype of all salts — have been ascertained by Witte and Wolf el (16) and by Scho-knecht (17). An electron density map in the (100) plane is shown in Fig. 1. The essentially ionic character of the crystal is confirmed and the density along the line Na—Cl drops to a low value1. From the electron distribution Witte and Wolfel derived radii of 1.17 A for Na+ and 1.64 A... 

It is of interest to consider the experimental radial electron density distribution in the ions Na+ and Cl- in sodium chloride in relation to corresponding results for the free ions calculated by the self-consistent field method. In Fig. 3 data from the experimental study of Schoknecht... 

The definition of crystal radii from the location of the minimum of the experimental electron density between neighbouring ions appears to be physically satisfactory when the individual ions approximate to spherical shape and show little overlap, as is the case in sodium chloride. Where deviations from spherical symmetry become more significant and the zone of electron cloud overlap is appreciable, the concept of ionic radius becomes dubious. 

Fig. 4.10 Electron den y contours m sodium chloride Numbers indicate the electron derahy (electrons — 10" electrons pm" ) along each contour line. The "boundary" of each ion is defined as the minimum in electron density bdween the ions. The Intcimidear distance is 2Sl pm (= Z8I A). [Modified frem Schoknecht. G. Z. Natuijarsck 1957,12A, 983. Reproduced with permisaoa]...

Use SpartanView to compare bond-density surfaces of methanol (CH3OH) and sodium methoxide (CH30Na). Which bond has higher electron density between the nuclei, 0-H or O-Na Repeat for acetic acid (CH3CO2H) and sodium acetate (CH3C02Na), and for hydrogen chloride (HCl) and sodium chloride (NaCl). How do your results relate to the covalent versus ionic nature of the bonds ... 

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