Lanthanides, Actinides, and Transuranium Elements

The lower half of most modern periodic tables contains two rows that seem to float apart from the rest of the elements. These two rows are the lanthanides and actinides. By atomic number, they should fall into Periods 6 and 7 of the table. Some extra-long tables do squeeze them into these periods. But the lanthanide and actinide elements have a few special features that truly set them apart from the rest of the transition metals. 

The lanthanide elements begin with lanthanum (atomic number 57) and go to ytterbium (atomic number 70). The actinide elements begin with actinium (atomic number 89) and go to nobelium (atomic number 102). 

The actinides include another part of the periodic table called the transuranium elements, which begin with neptunium (atomic number 93) and end with roentgenium (atomic number 111) back up in Period 7. Neptunium and plutonium are the only 

The lanthanides and actinides, along with the rest of the transuranium elements, are all metals. Although they are listed apart 

Like the transition metals, however, the lanthanides and actinides break the rules a little when it comes to their valence electron shell. Transition metals share electrons from the d orbital in their next-to-outermost shell. The valence electrons in lanthanides 

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For the lighter actinide elements, such as uranium, neptunium, plutonium, and americium, the promotion energy of 5f —> 6d is smaller than that of 4f 5d transition in the lanthanides. Due to this smaller promotion energy, the chemical bonding is complicated in the lighter actinides and these elements take from 3+ to 7+ oxidation states. However, the heavier actinides beyond curium more closely resemble the lanthanides and the trivalent state becomes stable. In order to understand the chemistry of transuranium elements, one has to consider the relative energy of the valence orbitals and the relativistic effects on... 

The lanthanide and actinide elements are located at the bottom of the periodic table in two rows separate from the rest of the elements. By atomic number, they should be located in Periods 6 and 7, but they have special properties that distinguish them from elements in those periods. Lanthanides are very similar to each other and have some industrial uses. Many of the actinides were discovered as part of the first atomic bomb experiments. They are highly radioactive and have few uses. The transuranium elements were mostly created in the laboratory and are very short-lived. 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

Figure 14.13. Ionic radii of actinide and lanthanide ions M- and M " as a function of the atomic number Z. (According to G. T. Seaborg The Transuranium Elements. Yale University Press 1958 Addison-Wesley Publ. Comp., S. 137.)...

The actinide iodate system is one of considerable interest that has attracted chemists for more than 150 years (vide supra). In fact one of the first forms that was isolated in was as the iodate salt, presumably as 1 0(103)4 [63], The precipitation of iodate compounds of the actinides has been used for decades as a method of separated them from lanthanides and other fission products. The precipitation of thorium iodate is perhaps best known in this regard [64-66], but several patents exist describing selective precipitation of transuranium elements [67-72], Despite the key importance of iodate in actinide chemistry the structures of actinide iodates were not described in detail until approximately 2000. 

The so-called rare earth elements, which are all metals, usually are displayed in a separate block of their own located below the rest of the periodic table. The elements in the first row of rare earths are called lanthanides because their properties are extremely similar to the properties of lanthanum. The elements in the second row of rare earths are called actinides because their properties are extremely similar to the properties of actinium. The actinides following uranium are called transuranium elements and are not found in nature but have been produced artificially. 

Freeman and Smith (32) have prepared the anhydrous chlorides of a number of lanthanides and of thorium by dehydrating the hydrated chlorides with thionyl chloride. Although efforts to obtain anhydrous plutonium trichloride in this way were unsuccessful, it is believed that this may be a useful procedure for actinide elements such as actinium, americium, and curium that have a particularly stable (III) oxidation stage. In general, aqueous methods for preparing tetrachlorides are of little value but anhydrous trichlorides, particularly of the transuranium elements, can be obtained readily from the hydrated trichlorides by dehydration in an atmosphere of hydrogen chloride. 

The true stars of Seaborg s 1944 Periodic Table are the transuranium elements neptunium (Np) and plutonium (Pu) as well as elements 89 to 92 (actinium, thorium, protactinium, and uranium). Neptunium was synthesized by McMillan and Abelson at Berkeley in 1940. In late 1940 and early 1941 McMillan, Kennedy, Wahl, and Seaborg made Pu through bombardment of uranium with deuterons in early 1941, and Pu was obtained by bombarding uranium with neutrons. It was Seaborg who, m 1944, proposed a new series of compounds for the Periodic Table—the actinides—analogous to the rare earths or lanthanides. In his book The Periodic Kingdom, Atkins describes the lanthanides... 

The actinides are a row of radioactive elements from thorium to lawrencium. They were not always separated into their own row in the periodic table. Originally, the actinides were located within the d-block following actinium. In 1944, Glenn Seaborg proposed a reorganization of the periodic chart to reflect what he knew about the chemistry of the actinide elements. He placed the actinide series elements in their own row directly below the lanthanide series. Seaborg had played a major role in the discovery of plutonium in 1941. His reorganization of the periodic table made it possible for him and his coworkers to predict the properties of possible new elements and facilitated the synthesis of nine additional transuranium elements. 

O Table 18.9 gives oxidation states of actinide elements (Katz et aL 1986). Actinium and transplutonium elements (from Am to Lr) take 3+ as the most stable oxidation state and they behave similar to the lanthanide elements, except element 102, No, which seems to prefer the 2+ state. Because of the itinerancy of the 5f electrons, the lighter actinide elements take broad range of oxidation states. The light transuranium elements, Np, Pu, and Am can behave as 3+ to 7+ cations and the most stable oxidation states of these three elements are 5+, 4+, and 3+, respectively. 

The redox reaction has been utilized in the separation of light actinide elements (U, Np, and Pu) with both ion-exchange process and solvent extraction process. For trivalent heavy actinides with Z> 94 (except No), separation of these actinide ions from lanthanide ions is required for safe storage of long-lived nuclear waste and transmutation of these nuclides. Fundamental researches have widely been carried out by several groups for the purpose of quantitative separation of transuranium elements. Recent topics on the development and application of solvent extraction for the separation of transuranium elements are briefly summarized below. 

Table 18.13 gives the crystal structures and phase transformations of the transuranium elements (Katz et al. 1986). The lighter actinide elements like Np and Pu have a bcc structure at the melting point and Pu has six distinct allotropic forms. The heavier actinides from Am to Es show face-centered cubic (fee) structures at the melting point and double hexagonal close-packed (dhep) structures below the melting point. These heavier actinides behave like lanthanide metals. 

The relativistic calculations on the electronic structure of actinide compounds were reviewed by Pyykko (1987). He also reviewed relativistic quantum chemistry in 1988, whereas the relativistic calculations were limited to small molecules containing one heavy atom only (Pyykko 1988). Calculations on the uranyl and neptunyl ions were introduced in the review article. The general information on the computational chemistry of heavy elements and relativistic calculation techniques appear in the book written by Balasubramanian (1997). There are several first-principle approaches to the electronic structure of actinide compounds. The relativistic effective core potential (ECP) and relativistic density functional methods are widely used for complex systems containing actinide elements. Pepper and Bursten (1991) reviewed relativistic quantum chemistry, while Schreckenbach et al. (1999) reviewed density functional calculations on actinide compounds in which theoretical background and application to actinide compounds were described in detail. The Encyclopedia of computational chemistry also contains examples including lanthanide and actinide elements (Schleyer et al. 1998). The various methods for the computational approach to the chemistry of transuranium elements are briefly described and summarized below. 

Element 94 was named plutonium after the planet discovered last, Pluto. In 1941, the first 0.5 /rg of the fissionable isotope Pu were produced by irradiating 1.2 kg of uranyl nitrate with cyclotron-generated neutrons. In 1948, trace amounts of Pu were found in nature, formed by neutron capture in uranium. In chemical studies, plutonium was shown to have properties similar to uranium and not to osmium as suggested earlier. The actinide concept advanced by G. T. Seaborg, to consider the actinide elements as a second / transition series analogous to the lanthanides, systematized the chemistry of the transuranium elements and facilitated the search for heavier actinide elements. The actinide elements americium (95) through fermium (100) were produced first either via neutron or helium-ion bombardments of actinide targets in the years between 1944 and 1955. 

The physicochemical properties of the actinide hydrides are as varied as any in the entire periodic table. Thorium forms a normal dihydride like those of Zr and Hf, but also forms Th4His, a unique superconductor. The hydrides of protactinium and uranium have cubic structures which have no counterparts in the periodic table. The transuranium element hydrides are more lanthanide like with wide cubic solid solution ranges. Hexagonal phases appear with regularity. 

The discovery of the next two transuranium elements, americium (Z = 95) and curium (Z = 96), depended on an understanding of the correct positions in the periodic table of the elements beyond actinium (Z = 89). It had been thought that these elements should be placed after actinium under the d-transition elements. So uranium was placed in Group VIB under tungsten. However, Glenn T. Seaborg, then at the University of California, Berkeley, postulated a second series of elements to be placed at the bottom of the periodic table, under the lanthanides, as shown in modem tables (see inside front cover). These elements, the actinides, would be expected to have chemical properties similar to those of the lanthanides. Once they understood this, Seaborg and others were able to use the predicted chemical behaviors of the actinides to separate americium and curium. 

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