Alkali metals ionic size

Figure 6.17. The lattice parameter ao as a function of the alkali-metal ionic radius in the T-site. The lattice size is well parameterized by the alkali-metal ionic radius in the T-site with little influence of it on the O-site. (Reproduced by permission from ref. 82.)...

The small size of lithium frequently confers special properties on its compounds and for this reason the element is sometimes termed anomalous . For example, it is miscible with Na only above 380° and is immiscible with molten K, Rb and Cs, whereas all other pairs of alkali metals are miscible with each other in all proportions. (The ternary alloy containing 12% Na, 47% K and 41% Cs has the lowest known mp, —78°C, of any metallic system.) Li shows many similarities to Mg. This so-called diagonal relationship stems from the similarity in ionic size of the two elements / (Li ) 76pm, / (Mg ) 72pm, compared with / (Na ) 102pm. Thus, as first noted by Arfvedson in establishing lithium as a new element, LiOH and LiiCOs are much less soluble than the corresponding... 

In each set, the atomic volumes increase going from halogen to inert gas to alkali metal, as shown graphically in Figure 6-9c. Figure 6-10 shows models constructed on the same scale to show the relative sizes of atoms indicated by the atomic volumes and by the packing of the ions in the ionic solids. 

Interestingly, more structures with a K-C than Na-C bond are known, despite the increasing reactivity of the compounds as descending the group of alkali metals. As frequently observed, K, Rb, and Cs compounds display somewhat similar chemistry, which typically differs from that of the lithium analogs. Sodium, in many instances, adopts a chemistry that resembles more that of lithium than the heavier congeners, most likely a function of the ionic size. 

These three structures are the predominant structures of metals, the exceptions being found mainly in such heavy metals as plutonium. Table 6.1 shows the structure in a sequence of the Periodic Groups, and gives a value of the distance of closest approach of two atoms in the metal. This latter may be viewed as representing the atomic size if the atoms are treated as hard spheres. Alternatively it may be treated as an inter-nuclear distance which is determined by the electronic structure of the metal atoms. In the free-electron model of metals, the structure is described as an ordered array of metallic ions immersed in a continuum of free or unbound electrons. A comparison of the ionic radius with the inter-nuclear distance shows that some metals, such as the alkali metals are empty i.e. the ions are small compared with the hard sphere model, while some such as copper are full with the ionic radius being close to the inter-nuclear distance in the metal. A consideration of ionic radii will be made later in the ionic structures of oxides. 

A number of useful properties of the Group 1 elements (alkali metals) are given in Table 8. They include ionization potentials and electron affinities Pauling, Allred-Rochow and Allen electronegativities ionic, covalent and van der Waals radii v steric parameters and polarizabilities. It should be noted that the ionic radii, ri, are a linear function of the molar volumes, Vm, and the a values. If they are used as parameters, they cannot distinguish between polarizability and ionic size. 

The diffusion of metal ions in vitreous silica has not been studied as extensively as that of the gaseous species. The alkali metals have received the most attention because their behavior is important in electrical applications. The diffusion coefficients for various metal ions are listed in Table 5. The general trend is for the diffusion coefficient to increase with larger ionic sizes and higher valences. 

The anionic catalysts listed earlier react with lactam monomer to first form the salt, which in turn will dissociate to the active species, namely, the lactam anion. A strongly dissociating catalyst in low concentrations, therefore, is always preferable to weakly dissociating catalysts in higher concentrations. The catalytic activity of the various alkali metal and quaternary salts of a lactam generally follows the extent of their ionic dissociation that is controlled by the cation. Activity of a salt decreases with increasing size of the cation due to restricted mobility and decreased ionization potential. 

The alkali metals share many common features, yet differences in size, atomic number, ionization potential, and solvation energy leads to each element maintaining individual chemical characteristics. Among K, Na, and Li compounds, potassium compounds are more ionic and more nucleophilic. Potassium ions form loose or solvent-separated ion pairs with counteranions in polar solvents. Large potassium cations tend to stabilize delocalized (soft) anions in transition states. In contrast, lithium compounds are more covalent, more soluble in nonpolar solvents, usually existing as aggregates (tetramers and hexamers) in the form of tight ion pairs. Small lithium cations stabilize localized (hard) counteranions (see Lithium and lithium compounds). Sodium chemistry is intermediate between that of potassium and lithium (see Sodium and sodium alloys). 

The superoxides are ionic solids containing the superoxide, Oj. Superoxides of all of the alkali metals have been prepared. Alkaline-earth metals, cadmium, and zinc all form superoxides, but these have been observed only in mixtures with the corresponding peroxides. The tendency to form superoxides in the alkali metal series increases with increasing size of the metal ion. 

Results obtained from studies on alkali fluoride and chloride crystals show that alkali metal ions (cesium in this case) dissolve first in the former, whereas chloride ions dissolve first in the latter. The differences are accounted for by the ionic sizes of the cation and anion, which are significantly different in the two cases. The dissolution of the second ion occurs soon afterward, although we cannot predict when. If the size of the cation and anion is about the same, the dissolution process is retarded. 

The ion-transfer feedback mode can also be used to probe the transfers of electroinactive ionic species, (e.g., CIO4), alkali metal cations, and tetra-alkylammonium cations across the ITIES. A micrometer- or nanometer-sized pipet can be filled with a solvent immiscible with the outer solution and used as a tip to approach a macroscopic ITIES (Fig. 4b). In an SECM study of... 

To obtain a picture of how loosely the valence electron in an alkali metal is held, consider two quantities connected with the most common of the alkali metals, sodium, the atomic radius arid the ionic radius. Now, one must be careful in speaking of the sizes of atoms or ions just as... 

---