Ionic sizes

Thus in the case of ions, measurements of this type are generally used to obtain values of the mobility and, through Stoke s law or related equations, an estimate of the effective ionic size. 

One of the major factors in determining the structures of the substances that can be thought of as made up of cations and anions packed together is ionic size. It is obvious from the nature of wave functions that no ion has a precisely defined radius. However, with the insight afforded by electron... 

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

Table 3 is arranged by crystal class (14). The crystal class of a given pigment is determined almost solely by the ratio of the ionic sizes of the cation and the anion and thek respective valences. Hence, for any given stoichiometry and ionic size ratio, only one or two stmctures will be possible. In some classes (spkiel, zkcon), a wide range of colors is possible within the confines of that class. Pigments within a given class usually have excellent chemical and... 

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... 

Different samples of aqueous solution containing radionuclides of Co and Eu were prepared at different copper sulphate concentrations and constant polymer concentrations (pAM) of 15 mg/1. The addition of salt to the system was done to reduce both the repulsion forces between the radionuclides and the interaction between the polymeric chains [7]. The polymer efficiency for the prepared samples was determined, results are shown in Fig. 15. It is clear that the polymer efficiency for Eu " is higher than for Co. This can be explained by the difference in the tightly bound structured water associated with different cationic species [14,107]. On this basis, we expect that Co is more hydrated than Eu. This is due to the difference in the ionic size. The hydra-... 

The large size of the unit of structure and the complexity of the chemical formula make the deduction of the atomic arrangement from X-ray data alone impractical if not impossible. We consequently make use also of arguments based on analogy with other structures, semi-empirical structural rules regarding ionic sizes and ionic environments, etc., with ultimate recourse to the stochastic1) method, which has already... 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

In our design, divalent Ca was chosen to partially substitute the trivalent atoms, and La and Ce were selected for a trivalent element because their ionic size (rLas+ = l.SOA rce3+=l-48A) was close to that of Ca (rca2+ = T48A) [21]. Like La, the Ce element also generally shows a formal -i-3 oxidation state in in-termetallics. Erom the reactions of the elements, we have identified as major phases the electron-precise/deficient alloys, Ln5.xCaxGe4 (Ln=La, Ce x=3.37,... 

The lattice energy is the sum of all Ion interactions, each of which is described by Equation. Looking at this equation, we can predict that lattice energy will increase as ionic charge increases and that it will decrease as ionic size Increases. A third trend occurs in the summing of all the ion contributions Lattice energy increases with the number of ions in the chemical formula of the salt. 

The number of water molecules in such a cluster, the hydration number, varies with ionic size it is four for Li, three for Na, but only one for Rb. ... 

Intense ion-ion interactions which are characteristic of salt solutions occur in the concentrated aqueous solutions from which AB cements are prepared. As we have seen, in such solutions the simple Debye-Hiickel limiting law that describes the strength goes up so the repulsive force between the ions becomes increasingly important. This is taken account of in the full Debye-Hiickel equation by the inclusion of a parameter related to ionic size and hence distance of closest approach (Marcus, 1988). 

Because of its ionic size, cadmium(II) does not fit well into the coordination site of the porphyrin nucleus (N-centre distance 200 pm), and the complexes are readily demetalated (stability group IV, Tables land 2). The complexes can be prepared, of course, but zinc(II) compounds have been overwhelmingly favored for PDT purposes. 

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. 

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