Actinide elements lanthanides compared with

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

The ilillerent ranges of oxidation stales of the actinides in aqueous solution were described, discussed and compared with the much narrower ranges displayed by the lanthanide elements. 

For the lighter chemical elements, the velocity of the electrons is negligible compared with the velocity of light. However, for the actinides and to a lesser extent the lanthanides this is not the case as the velocity of the electrons increases towards c, then their mass increases too. 

The chemistry of the lighter actinides from thorium to americium, all being available in substantial quantities, is now well understood. In the - -4, or higher oxidation states, these elements are best considered as an inner transition series. Their chemistry shows both horizontal similarities within the actinide group and to a lesser degree, some vertical similarities with the group 4, 5, and 6 d-transition elements. All of the actinides in their +3 oxidation states behave in much the same way as the lanthanides. The chemistry of the actinides is reviewed within this context and compared with the corresponding lanthanides. 

A comparable temperature dependence of resistivity was obtained for a mixed phase sample of Cel2. The formation of apparently metallic phases for only the iodides of five lanthanide and actinide elements is considered in terms of the stoichiometry, the electronic structure of the cation, the possible nature of the band, and the role of the anion. In contrast, the intermediate Lai2.1,2 phase exhibits semiconduction. Its magnetic data between 80° and 300° K. can be best accounted for if the reduced component is considered to be La ", [Xe]5d with a ground term, a spin-orbit coupling constant A — 050 cm. and only small covalency and asymmetry parameters. 

These ionic radii, derived from X-ray diffraction data, should be compared with those of the lanthanides (p. 421). The size of an ion depends largely upon the quantum number of the outermost electrons and the effective nuclear charge (p. 89). In the 3+ ions of these elements the outermost electrons are in a completed 6p shell the effective nuclear charge rises with atomic number because the screening effect of extra electrons in the 5f level fails to compensate entirely for the increased nuclear charge. The existence of a contraction, similar to the lanthanide contraction, affords further support for the idea that the 5f level is being filled in passing onwards from actinum. The contraction is more rapid in the actinides. 

Compared with the lanthanides or the transition metals, the actinide elements introduce a striking array of novel chemical features, displayed most clearly in the chemistry of uranium. There is the variety of oxidation state, and to some extent the chemical diversity, typical of transition metals in the same periodic group, but physical properties which show that the valence electrons occupy /-orbitals in the manner of the lanthanides. This raises the question of the nature of the chemical bond in the compounds of these elements. The configuration of the uranium atom in the gas phase is f3ds2, so it is natural to ask whether there are special characteristics of the bonding that reflect the presence of both/and d valence orbitals. 

The dilTcrcnl raiiiies ol oxidation stales ol lire actinides in a(. ueous solution weie dcscribal. discussal aiul compared with the much narrower laimes tiisplayed b the lanthanide elements. 

The actinide elements undergo bonding very differently from the lanthanides due to the difference in the spatial extension of the frontier orbitals compared with the inner electron orbitals. 

It is our purpose to present a summary of the compounds formed with oxygen and the lanthanide and actinide elements in such a way as to facilitate comparisons and contrasts. It is hoped that for many readers this summary will, in itself, prove useful by presenting an updated account of some relevant research concerning the lanthanide and actinide oxides. Beyond this, it will be found that the oxides afford a substantial platform upon which to assemble a comparative collage of lanthanide/ actinide chemistry. 

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. 

A review of the thermodynamic properties of the actinides and selected comparative data for the lanthanides have been given (Morss 1986). One experimental approach that has been frequently used for obtaining enthalpies of formation of the oxides has been through solution calorimetry, and many data have been acquired for the actinide oxides through Cf in the series. Elements with a higher Z than Cf are not amenable to this technique. 

Curium is a lustrous, malleable, silvery metal with many properties quite comparable to those of the lighter actinide elements. The melting point of Cm (dhcp form)is 1345 50°C [71], much higher than for the immediately preceding actinide elements, Np-Am (639-1173 C), but very similar to that of gadolinium (1312°C), its lanthanide analog [27,162]. 

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