Alkali metal halides ionic radii

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

All the alkali metal halides except the cliloride, bromide and iodide of caesium form cubic crystals with the rock salt lattice and show a co-ordination number of 6. The exceptions are also cubic, but have the caesium chloride structure (Fig. 133) characterised by a co-ordination number of 8. The radius ratio for CsCl, Cs /Cl" = 0.93, allows 8 co-ordination, but is so near the ratio for 6 co-ordination that caesium chloride is dimorphous, changing, at 445°, from the caesium chloride to the rock salt structure. The crystalline halides are generally markedly ionic, though, as expected, lithium iodide is somewhat covalent, for iodide is the largest and most easily polarised simple anion and lithium, the smallest alkali metal cation, possesses the strongest polarising power. 

Electrical conductivity of molten alkali and earth alkali metal halides increase by 2 orders at melting. The electrical conductivity of these melts is purely ionic and their electrolysis follows Faraday s law. Deviations from this law are caused by secondary processes during the electrolysis, as for example dissolution or the back reaction of the electrolysis products. Electrical conductivity and thus also the mobility of ions is, in general, given by quantities like ionic charge, ionic mass, radius, polarizability, and the coordination number. 

For oxides of the type MeO, there exists a linear dependence of the solubility product index of the oxide against the inverse-squared radius of the metal cation. The slopes of these plots in the melts based on alkali-metal halides and alkaline-earth metal halides are stated to be approximately the same. This can give evidence that, at high temperatures in the order of 1000 K, the changes in solvation ability of the ionic melts, proceeding from one melt to another, are close for different cations with radii in the order of 0.1 nm (from 0.74 nm for Mg2+ to 1.38 nm for Ba2+). An increase in the melt temperature... 

Pauling considered a series of alkali metal halides, each member of which contained isoelectronic ions (NaF, KCl, RbBr, Csl). In order to partition the ionic radii, he assumed that the radius of each ion was inversely proportional to its actual nuclear charge less an amount due to screening effects. The latter were estimated using Slater s rules (see Box 1.6). 

Fig. 12.2 The ratio of radii, k (=ionic radius/covalent radius), for alkali metal eations (M ) and halide anions (X ) in aqueous solutions (Eqs. 12.6a, b). In the right angled triangle, ABC, E and D are the mid points of AB and AC...

The alkali metals react with many other elements directly to make binary solids. The alkali halides are often regarded as the most typical ionic solids. Their lattice energies agree closely with calculations although their structures do not all conform to the simple radius ratio rules, as all have the rock salt (NaCl) structure at normal temperature and pressure, except CsCl, CsBr and Csl, which have the eight-coordinate CsCl structure. The alkali halides are all moderately soluble in water, LiF being the least so. 

Molar ionic conductivity — This quantity, first introduced by -> Kohlrausch, is defined by A = Zi Fui (SI unit Sm2 mol-1), where Zj and 14 are the charge number and -> ionic mobility of an ion, respectively. The molar -> conductivity of an electrolyte M +X (denoted by A) is given by A = u+X+ + i/ A, where A+ and A are the molar ionic conductivities of the cation and anion. The A value of an ion at infinite dilution (denoted by A°°) is specific to the ion. For alkali metal ions and halide ions, their A values in water decrease in the orders K+ > Na+ > Li+ and Br- > Cl- > F-. These orders are in conflict with those expected from the crystal ionic radii, because the smaller ions are more highly hydrated, so that the -> hydrated ions become larger and thus less mobile. Based on Stokes law, the radius of a hydrated ion... 

Fig. 3.2 Plots of —z((AsGi) against ionic radius r for the alkali metal cations ( ), alkaline earth metal cations ( ), halide anions (a) and the sulhde anion (t). The parameters 5s and/dd are determined from the intercept and slope of these plots, respectively, according to equation (3.5.14). The plot for cations has been shifted vertically by 1 mol MJ for the sake of clarity.

On looking into the literature to see whether this value of d(H ) = 0.28 A is meaningful, it was found [1] that this is the value suggested by Pauling [8] to account for the radius of H in the partially ionic bonds in hydrogen halides and in alkali metal hydrides The author then used this value of d(H ) = 0.28 A to estimate the radii of alkali metal ions from the observed bond distances d(MH) g in the metal hydrides, MH. It was a pleasant surprise to find that,... 

In this category, we only have results for alkali halides. After Grimley in LiF, the net charges are very close to +1 and -1. In Table 4, we indicate after the increasing electron charge carried by the metal in alkali halides. These values were obtained by numerical integration of the electron density contained inside a sphere radius equal to the classical ionic radius. 

Three particles have the same electron configuration. One is a cation of an alkali metal, one is an anion of the halide in the third period, and the third particle is an atom of a noble gas. What are the identities of the three particles (including charges) Which particle should have the smallest atomic/ionic radius, which should have the largest, and why ... 

Three years later, Wasastjema was able to present a table with more accurate radii for 16 ions, including 0 , F, Na", and Mg" (Table 3) [36]. Wasastjema fixed the ratio between the radius of an alkali metal cation and the radius of the isoelectronic halide anion (for instance, the ratio between the radii of Na" and F ) using polarizability data from aqueous solutions. The polarizabilities of the halide anions were first determined under the assumption that the hydrogen ions do not contribute significantly to the molar polarizabilities of dissolved hydrogen halides. The polarizabilities of the cations could then be obtained from measurements on the salts. Molar polarizabilities are roughly proportional to the ionic volumes, and Wasastjema fixed the radius ratio r+/r as the fourth root of the polarizability ratio of the two ions (Table 3). 

The ionic hydration energies (enthalpies and Gibbs free energies) of metals are consequently roughly a linear function of the square of the oxidation state divided by the effective ion radius (z /r ff. Figure 8.9).It may be added that AH y and AG y, respectively, of the individual alkali halide ion pairs form two linear branches with a maximum at Csl (KBr). 

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