Potassium metallic radius, 178-9

The viscosities of liquid metals vaty by a factor of about 10 between the empty metals, and the full metals, and typical values are 0.54 x 10 poise for liquid potassium, and 4.1 x 10 poise for liquid copper, at dreir respective melting points. Empty metals are those in which the ionic radius is small compared to the metallic radius, and full metals are those in which the ionic radius is approximately the same as tire metallic radius. The process was described by Andrade as an activated process following an AiThenius expression... 

A host of carriers, with a wide variety of ion selectivities, have been proposed for this task. Most of them have been used for the recognition of alkali and alkaline metal cations (e.g., clinically relevant electrolytes). A classical example is the cyclic depsipeptide valinomycin (Fig. 5.13), used as the basis for the widely used ISE for potassium ion (38). This doughnut-shaped molecule has an electron-rich pocket in the center into which potassium ions are selectively extracted. For example, the electrode exhibits a selectivity for K+ over Na+ of approximately 30,000. The basis for the selectivity seems to be the fit between the size of the potassium ion (radius 1.33 A) and the volume of the internal cavity of the macrocyclic molecule. The hydrophobic sidechains of valinomycin stretch into the lipophilic part of the membrane. In addition to its excellent selectivity, such an electrode is well behaved and has a wide working pH range. Strongly acidic media can be employed because the electrode is 18,000 times more responsive to K+ than to H+. A Nernstian response to potassium ion activities, with a slope of 59mV/pK+, is commonly observed... 

Potassium adopts the A2 stmcture, with a lattice parameter of 0.5321 nm. Estimate the metallic radius of potassium in this stmcture. 

PRACTICE EXAMPLE A Potassium crystallizes in the bcc structure. What is the length of the unit cell in this structure Use the metallic radius of potassium given in Figure 9-11. 

The principal product of the reaction of the alkali metals with oxygen varies systematically down the group (Fig. 14.15). Ionic compounds formed from cations and anions of similar radius are commonly found to he more stable than those formed from ions with markedly different radii. Such is the case here. Lithium forms mainly the oxide, Li20. Sodium, which has a larger cation, forms predominantly the very pale yellow sodium peroxide, Na202. Potassium, with an even bigger cation, forms mainly the superoxide, K02, which contains the superoxide ion, O,. ... 

In other crystals an octahedral metal atom is attached to six non-metal atoms, each of which forms one, two, or three, rather than four, bonds with other atoms. The interatomic distance in such a crystal should be equal to the sum of the octahedral radius of the metal atom and the normal-valence radius (Table VI) of the non-metal atom. This is found to be true for many crystals with the potassium chlorostannate (H 61) and cadmium iodide (C 6) structures (Table XIB). Data are included in Table XIC for crystals in which a tetrahedral atom is bonded to a non-metal atom with two or three covalent bonds. The values of dcalc are obtained by adding the tetrahedral radius for the former to the normal-valence radius for the latter atom. 

The curve of single-bond metallic radii for the elements of the first long period has a characteristic appearance (Fig. 3) which must be attributed in the main to variation in the type of bond orbital. The rapid decrease from potassium to chromium results from increase in bond strength due to increasing s-p and d-s—p hybridization. The linear section of the curve from chromium to nickel substantiates the assumption that the same bonding orbitals (hybrids of 2.56 3d orbitals, one 4s orbital, and 2.22 4p orbitals) are effective throughout this series. The increase in radius from nickel to copper is attributed not... 

If we take a series of alkali metal halides, all with the rock salt structure, as we replace one metal ion with another, say sodium with potassium, we would expect the metal-halide internuclear distance to change by the same amount each time if the concept of an ion as a hard sphere with a particular radius holds true. Table 1.8 presents the results of this procedure for a range of alkali halides, and the change in internuclear distance on swapping one ion for another is highlighted. 

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. 

Because alkoxy substituted alcohols are acid sensitive the support must be either neutral or even basic. That the presence of a basic promoter is needed can be seen by a strongly decreased activity when the Ca and Mg, which are present originally in the carrier, are removed by washing with 2N HC1. Catalysts prepared with additional basic metals as chlorides (1% metal on catalyst) were tested at 200°C. At these temperatures there are only very small differences between the metals. Figure 2 shows a plot of ionic radius versus conversion of MEA. The best promotors are potassium and calcium. 

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 ... 

The allyl lariat ether (Section 3.3) 3.132 forms complexes with both K+ and Ag+ (which are of similar ionic radius, Table 3.5). In the case of the K+ complex, as may be expected, the potassium ion is too large to fit snugly within the aza[15] crown-5 ring and lies somewhat above the donor atom plane. The exposed face of the metal atom is occupied by a PF6" anion. Similar coordination to the crown is exhibited by Ag+ but instead of an anion, the exposed face of the metal ion is coordinated to the allyl side chain of an adjacent molecule to give an infinite polymer in the solid state (Figure 3.91). 

Barium reacts with metal oxides and hydroxides in soil and is subsequently adsorbed onto soil particulates (Hem 1959 Rai et al. 1984). Adsorption onto metal oxides in soils and sediments probably acts as a control over the concentration of barium in natural waters (Bodek et al. 1988). Under typical environmental conditions, barium displaces other adsorbed alkaline earth metals from MnO2, SiO2, and TiO2 (Rai et al. 1984). However, barium is displaced from Al203 by other alkaline earth metals (Rai et al. 1984). The ionic radius of the barium ion in its typical valence state (Ba+) makes isomorphous substitution possible only with strontium and generally not with the other members of the alkaline earth elements (Kirkpatrick 1978). Among the other elements that occur with barium in nature, substitution is common only with potassium but not with the smaller ions of sodium, iron, manganese, aluminum, and silicon (Kirkpatrick 1978). 

Fig. 4. Hartree-Fock free atom 4s valence electron orbital for potassium (solid line) and the 4s-like orbital, obeying the Wigner-Seitz boundary condition, appropriate to the bottom of the conduction bands in metallic potassium (dashed line). Both orbitals are normalized, for the metal, integration is limited to the Wigner-Seitz sphere of radius rws...

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