Lithium ionic radii

With the knowledge now of the magnitude of the mobility, we can use equation A2.4.38 to calculate the radii of the ions thus for lithium, using the value of 0.000 89 kg s for the viscosity of pure water (since we are using the conductivity at infinite dilution), the radius is calculated to be 2.38 x 10 m (=2.38 A). This can be contrasted with the crystalline ionic radius of Li, which has the value 0.78 A. The difference between these values reflects the presence of the hydration sheath of water molecules as we showed above, the... 

The physical picture in concentrated electrolytes is more apdy described by the theory of ionic association (18,19). It was pointed out that as the solutions become more concentrated, the opportunity to form ion pairs held by electrostatic attraction increases (18). This tendency increases for ions with smaller ionic radius and in the lower dielectric constant solvents used for lithium batteries. A significant amount of ion-pairing and triple-ion formation exists in the high concentration electrolytes used in batteries. The ions are solvated, causing solvent molecules to be highly oriented and polarized. In concentrated solutions the ions are close together and the attraction between them increases ion-pairing of the electrolyte. Solvation can tie up a considerable amount of solvent and increase the viscosity of concentrated solutions. 

A similar reasoning may explain the difference in reactivities of the lithium and sodium ion-pairs in THF. The larger ionic radius of the sodium than that of the lithium cation, favoring the formation of loose pairs, makes the sodium pair much more reactive than the lithium salt at lower temperatures. However, at higher temperatures the sodium salt becomes less reactive than the lithium salt as it looses its solvation more readily than the latter. 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

Thirdly, there is the purely structural argument from Relative Size if ions of one type are much the largest, they will effectively fix the structure since the others can pack between them. This argument, which makes no assumption whatever about electron-clouds, is often referred only to lithium iodide, but much more evidence is available. Such questions of crystal-form and isomorphism are in fact the most important applications of ionic-radius systems in chemistry and mineralogy (cp. the classical work of V. M. Goldschmidt (2)). 

The ability of a metal alcoholate to accommodate an additional molecule of carbohydrate increases with increasing ionic radius " Li < Na < K < Cs. The difference in stoichiometry between lithium and sodium is much greater than that between either sodium and potassium, or potassium and cesium. The coordination number of an alkali metal is known to increase with increasing ionic radius. Brewer148 reported that the maximum number of donor groups oriented about an alkali metal cation is four for lithium, and as many as six for sodium, potassium, rubidium, or cesium. A greater surface area would allow accommodation of more than one carbohydrate moiety but, in addition, solvent molecules are more strongly attached to cations of smaller radius, and these may not be readily displaced by carbohydrate molecules. 

Li20(s). The other members of the group form mainly the peroxide or superoxide. Lithium exhibits the diagonal relationship that is common to many first members of a group. Many of lithium s compounds are similar to the compounds of Mg. This similarity is related to the small ionic radius of Li+, 58 pm, which is close to the ionic radius of Mg24, 72 pm, but substantially less than that of Na+, 102 pm. 

However, a careful study of the experimental data has led to some general trends. For instance, the nature of the final products depends heavily on the alkali cations used in the starting compounds sodium and lithium phenoxides reacting under similar experimental conditions yield the related salicylates as major products [18] (Scheme 5.1), whereas potassium, rubidium, and cesium phenoxides yield mixtures of 2-hydroxy-benzoic acid and 4-hydroxy-benzoic acid [1] (Scheme 5.2). As a rule of thumb, the yield of p-hydroxybenzoic acid generally increases with the increasing ionic radius of the alkali metal. Both, temperature and C02-pressure were also reported to be paramount in the selectivity of the carboxylation ... 

Lithium iodide crystallizes in the NaCl lattice in spite of the fact that r+/r is less than 0.414. Its density is 3.49 g/cm3. Calculate from these data the ionic radius of the iodide ion. 

The ionic radius of lithium is 0.78 A whereas that for beryllium is 0.34 A. If the positive charge be considered as spread over the ion, calculate the ratio of ike charge-per-unit-Yoiume values of the two ions. 

The interionic distance in lithium fluoride is found to be 2.01 A subtracting from this figure the ionic radius of F" (1.36 A obtained above), one may fix the ionic radius of Li+ as about 0.65 A. However, if estimates of the interionic distances in the remaining lithium halides are made by adding 0.65 A to the respective halide radii, the sums so obtained are about 10 percent less than the observed values (Table 12—1). Similarly... 

Lithium has two stable isotopes, Li and Li, which have abundances of 7.5% and 92.5%, respectively. Lithium is a soluble alkali element. Because its ionic radius is small (0.78 A), it behaves more like magnesium (0.72 A) than the alkalis. Li tends to substitute for Al, Fe, and especially for Mg " ". Because of their large relative mass difference, lithium isotopes have the potential to exhibit sizable fractionation, as has been demonstrated by high-precision isotopic analysis. 

The alkali metal sodium, having a larger ionic radius than lithium, prefers a pentacoordinated state. This can be accomplished by using the tridentate ligand PMDTA instead of TMEDA in the crystallization experiments. Thus, the adduct [PMDTA Na]2Mg(/z-Ph)4 (69) was isolated (36),... 

Lithium Trace amounts of lithium, the lightest alkali metal, are found in water, soil, and rocks. Lithium is the least reactive of the alkali metals. Its compounds are less likely to dissolve in water. In these and other properties, lithium is more closely related to magnesium than to the other alkali metals. Lithium has an atomic radius of 152 pm and an ionic radius of 76 pm. Magnesium has an atomic radius of 160 pm and an ionic radius of 72 pm. These similar physical properties lead to similar chemical properties, which is why lithium and magnesium have a diagonal relationship. 

Although there is a general increase in c.n. s of cations with increase in ionic radius a detailed correspondence between c.n. and radius ratio is not observed for simple ionic crystals. For example, all the alkali halides at ordinary temperature and pressure except CsCl, CsBr, and Csl crystallize with the NaCl structure. For Lil and LiBr (and possibly LiCl) the radius ratio is probably less than 0-41, but the radius ratios for the lithium halides are somewhat doubtful because the interionic distances in these crystals are not consistent with constant (additive) radii ... 

Lithium bromide, LiBr, crystallizes in the NaCl face-centered cubic strucmre with a unit cell edge length oi a = b = c = 5.501 A. Assume that the Br ions at the corners of the unit cell are in contact with those at the centers of the faces. Determine the ionic radius of the Br ion. One face of the unit cell is depicted in Figure 13-30. 

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