Actinides comparison with lanthanides

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

In comparison with the lanthanides, the actinides show a more complex dependence of several properties on the atomic number an analysis, for instance, of the... 

Since plutonium is the actinide generating most concern at the moment this review will be concerned primarily with this element. However, in the event of the fast breeder reactors being introduced the behaviour of americium and curium will be emphasised. As neptunium is of no major concern in comparison to plutonium there has been little research conducted on its behaviour in the biosphere. This review will not discuss the behaviour of berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium which are of no concern in the nuclear power programme although some of these actinides may be used in nuclear powered pacemakers. Occasionally other actinides, and some lanthanides, are referred to but merely to illustrate a particular fact of the actinides with greater clarity. 

For actinides heavier than Cm, a very similar scheme is worked out consisting in a comparison with a) trivalent lanthanides b) surely divalent lanthanides Eu and Yb. In it, Ecoh (trivalent) calculated with the above interpolation scheme, are compared with Ecoh for divalent metals, as obtained by assuming a behaviour across the actinide series, similar to the one found in divalent lanthanides. The divalency of the heavier actinides (and the trivalency of Am and Cm) is concluded. 

K. A. Gschneidner Jr., L. R. Eyring, G. R. Chopin, and G. A. Lander, Eds., Handbook of Physics and Chemistry of Rare Earths, Vol. 18, Elsevier-North Holland, Amsterdam, 1994. Comparison of lanthanides with actinides. 

Trihalides are known for most of the actinides and form the basis for comparison with the lanthanides. Table 10.2 lists the known structures adopted by the trihalides. 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

From the results shown in Fig. 6, some conclusions may be drawn. A comparison of the lanthanide curve with the summary curve for actinides suggests that the doubledouble effect is more pronounced in actinides than in lanthanides. A comparison of curves 3 and 4 reveals that the effect is the stronger, the higher the oxidation state of the actinide ions. Moreover, curves relating to actinides show, that the mean specific unit cell volumes decreases with Z more rapidly for this series than for lanthanides. This suggests that the actinide contraction is larger than that of lanthanides. Such a conclusion also results from the data of Zachariasen64). 

Fig. 57. Comparison of the elution data for some trivalent lanthanides with their actinide counterparts with ammonium lactate (90). Note the linear correlation within the second and third tetrads reproducing half of the Inclined W .

In the following, methods for preparation, purification and characterization of actinide metals are reviewed. Properties are presented, the theoretical interpretation of which underlines the special nature of the actinides in comparison with d or 4f (lanthanide) transition metals. 

The separation of No from other actinide elements is based entirely on the dissimilar behavior of No + in comparison with the tripositive actinide ions. Without the addition of strong oxidants, No will be present as No + in acidic solutions and will have the general chemical properties of Group IIA elements in the Periodic Table. We have found that the extraction chromatographic method described in the section on Md provides an effective separation from all other actinides and lanthanides. In contrast to Md, reducing agents are unnecessary in separating No by this extraction chemistry. 

The first measurement of the temperature dependence of an optical line width in an actinide system, Np + in LaC, was recently completed (47). The fluorescence transitions at 671.4 and 677.2 nm were studied from 10 to 200 K. The low temperature limit for the line width of the 677.2 nm transition is 16.5 GHz and is a measure of the width of the first excited crystal-field level of the ground manifold. The 671.4 nm transition has a line width of 0.55 GHz at 10 K. Its temperature dependence is described in terms of an effective three-level scheme for the excited manifold. The parameters are comparable to those found for Pr + in LaF. Further comparison depends upon the details of the phonon spectrum and the electronic states. At low temperatures, the residual width of the 671.4 nm transition was limited by the laser line width. This is consistent with the very narrow line widths observed in Pr +. Additional detailed studies of this type and proper contrast and comparison between lanthanides and actinides may provide the additional information needed to describe the electron-phonon and electron-ligand interactions of the actinides. 

By detailed comparison with the elution of lanthanide ions and by extrapolating data for the lighter actinides such as Np3+ or Pu3 +, the order of elution of the heavier actinides can be accurately forecast. Even a few atoms of the element can be identified because of the characteristic nuclear radiation. 

The hydration entropy for the ion A y8S°(M" ) represents the standard entropy change (usually at 298K) for the process M" (g)-> M" (aq). This property should reflect lanthanide-actinide differences because the final state represents the ion with all the water molecules in the primary and outer hydration spheres. Bratsch and Lagowski (1985b, table I) proposed a set of hydration parameters Ay and by which hydration entropies could be calculated for the lanthanides. Rizkalla and Choppin (1991, table 11) used these parameters to tabulate entropies of hydration for the lanthanides. However, it is not reasonable to extend these entropies for a lanthanide-actinide comparison because there are no experimental data from which independent actinide hydration entropy parameters Ay and can be calculated (see section 2.2.2 for experimental entropies of aqueous ions). 

Some magnetic properties of the lanthanides are presented in section 3 in comparison with the actinides. Table 33 and fig. 27 should be consulted for specific electronic configurations and magnetic moments in each series. 

The objective of this section of the chapter is to compare the properties and behaviors of the binary oxides of the lanthanide and actinide elements. The trends and the differences between the binary oxides of each series of elements are reviewed but a discussion of the more complex (e.g., ternary or larger) oxides that these elements are known to form is excluded. Essentially this section offers a comparison of the monoxides, sesquioxides, dioxides and binary oxides with 0/M ratios intermediate to those found in these three oxides. Since the lanthanide elements do not form oxides with higher O/M ratios than 2.0, actinide oxides with higher oxygen stoichiometries are not discussed in this section. 

Further scaling rules with respect to the delocalization of the f electrons can only be derived by comparison of the lanthanide and actinide metals with each other, and are therefore postponed to the next section. 

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