Actinide elements orbitals

As the computational effort in the LDF approach grows, in the limit, only with the third power in the number of orbitals, it can be expected that fairly large systems with a hundred atoms, including transition metals, rare earth, and actinide elements, will become tractable. 

It was Seaborg who proposed the actinide concept it has become generally accepted practise to call the 15 elements beginning with Ac the actinide elements. However, we have to notice that, although immediately following lanthanum the 4f orbitals become more favourable than the 5d for the entrance of further electrons in the following elements there is not (until later) a similar situation for the 5f-6d... 

The actinide elements result from the Ailing up of the bf orbitals and are the analogs of the lanthanides (J). There is a close similarity between the trivalent 4/and 5/ions. However, the actinides differ from the lanthanides in... 

Brooks and Johansson , by calculating I and AE have been able to show that these curves are very much affected by the spin-polarization energy AE, which, in fact, appears as the most important contribution in cohesion for actinide elements in which the beginning of localization of 5 f orbitals occurs (at the half-filling of the 5 f" shell) and for which spin-polarization plays a major role (see Chaps. C and F). 

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

The elements are often placed into categories on the basis of their electron configurations the transition elements where d orbitals are being filled and the lanthanides and actinides where /orbitals are being filled the remainder of the elements are often termed representative elements. 

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... 

Unlike the 4/orbitals in the lanthanides, the 5/orbitals in the earlier actinide elements are more expanded and so can be engaged in chemical bonding. This leads to a pattern of chemistry more analogous to that found in the d block, with the possibility of variable oxidation states up to the maximum possible determined by the number of valence electrons. Most thorium compounds contain Th(IV) (e.g. Th02) and... 

As a result of this disparity, many questions on the structure and bonding of the actinides center on the role of the 5f-electrons. The molecular orbital descriptions for the bonding of the actinide elements continue to evolve. One of the first general models used to describe the chemical bonding in d- and f-electron complexes is the FEUDAL model. FEUDAL is an acronym for f orbitals essentially unaffected d orbitals accommodate ligands . This model is represented in Figure 3, which depicts the molecular orbital... 

Not every actinide element has known or well-developed organometallic chemistry. By far the most research has been done on thorium and uranium compounds, a consequence of favorable isotope-specific nuclear properties and, at least until recently, the commercial availability of key starting materials such as Th metal, anhydrous ThCLi, U metal, and anhydrous UCL. Thorium chemistry is dominated by the -F4 oxidation state and has some similarities to the chemistry of the heavier group 4 metals. For uranium, one can access oxidation states from d-3 to 4-6 in organic media. Although there are some similarities to the chemistry of the heavier group 6 elements, for example, tungsten, there are also some remarkable differences made possible by the availability of the 5f valence orbitals. 

Continuing this pattern, the first and second lEs of the actinides are less sensitive to increasing atomic number than the third lEs (Figure 2.6). However, owing to the similarity in energies between the 6d and 5f subshells, the behaviour of the third lEs is less simple than for the lanthanides, although maxima do appear at Am and No ". The behaviour of the actinide elements is also complicated by the effects of relativity. These result in a contraction of the 7s and 7p orbitals but an expansion and destabilization of the 6d and 5f orbitals. As a consequence, the actinide valence shell 6d and 5f electrons are more easy to ionize than would be predicted by a non-relativistic model. 

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. 

Actinides show a greater multiplicity of oxidation states. The 5/orbitals extend into space beyond the 6s and 6p orbitals and participate in bonding, while 4/orbitals are totally shielded by outer orbitals and thus unable to take part in bonding. The participation of the 5/orbitals explains the higher oxidation states shown by the earlier actinide elements. The greater extension of 5/orbitals compared-with 4/is shown by the difference in electron resonance spectra. Since in the first half of the actinide series i.e., lower actinides), the energy required for the conversion of 5/ 6d is less than that required for the conversion of 4/ 5d, the lower actinides should, on a comparative basis, show more higher oxidation states such as + 4, + 5, + 6 and + 7. 

Every element, of course, has a different and unique number of electrons. But most of the additional electrons required by successively heavier actinide elements are added in an inner ring of the atom, a ring of electrons known as the 5f orbit. 

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. 

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