Properties of the actinide elements

In the chemistry of the fuel cycle and reactor operations, one must deal with the chemical properties of the actinide elements, particularly uranium and plutonium and those of the fission products. In this section, we focus on the fission products and then chemistry. In Figures 16.2 and 16.3, we show the chemical composition and associated fission product activities in irradiated fuel. The fission products include the elements from zinc to dysprosium, with all periodic table groups being represented. 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

By virtue of the fact that the actinide elements constitute a family of elements whose electronic configurations are related to each other in a quite unique way, a comparison of the physical and chemical properties of a series of related compounds has more than the usual interest. In this section some of the more important properties of the actinide element halides are summarized in tabular form, and a number of topics of current interest are described. 

Ward, J. W., Kleinschmidt, P. D., Peterson, D. E., Thermochemical properties of the actinide elements and selected actinide-noble metal intermetallics, Handbook on the physics and chemistry of the actinides, Freeman, A. J., Keller, C., Eds., vol. 4, pp.309-412, North-Holland, Amsterdam, (1986). Cited on pages 88, 89, 91, 92, 594. 

Additional electrons can fall into either of the d, f, or s orbitals, depending on which has the lowest energy requirement. This causes some fluctuation in the chemical and physical properties of the actinide elements. 

All of the actinide elements are metals with physical and chemical properties changing along the series from those typical of transition elements to those of the lanthanides. Several separation, purification, and preparation techniques have been developed considering the different properties of the actinide elements, their availability, and application. Powerful reducing agents are necessary to produce the metals from the actinide compounds. Actinide metals are produced by metallothermic reduction of halides, oxides, or carbides, followed by the evaporation in vacuum or the thermal dissociation of iodides to refine the metals. 

The first edition consisted of a serial description of the individual actinide elements, with a single chapter devoted to the six heaviest elements (lawrendum, the heaviest actinide, was yet to be discovered). Less than 15 % of the text was devoted to a consideration of the systematics of the actinide elements. In this edition nearly half of the work consists of survey chapters in which such subjects as the metallic state, thermochemistry, solid state chemistry, solution chemistry, atomic and electronic spectroscopy, magnetic properties, organometallic chemistry, and the biological and environmental properties of the actinide elements are treated in comparative fashion. Because of the expansion of the discipline and of the scope of the second edition, many cdleagues were asked to contribute chapters that reflected their expert knowledge. 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

The toxicity of the actinide elements which requires an absolute barrier between the experiment and the experimenter is dictated to only a small extent by external radiation hazards. Plutonium-239 is intensely radioactive, emitting 1.4 X 10 a particles per milligram per minute. However, the alpha radiation from plutonium-239 can easily be shielded by even a thin sheet of paper. It is the consequences of ingestion that make plutonium-239 and the other actinide elements such toxic substances. Plutonium-239, inhaled into the lungs as fine particulate matter, is translocated to the bone, and, over a period of time, may give rise to bone neoplasms (cf. Section 14.10). The biological properties of the actinide elements are discussed in more detail in Sections 14.9 and 14.10. 

A number of the properties covered in this chapter are also given elsewhere in this volume. In this presentation, the properties of the actinide elements are taken as a whole and attempts are made to compare them with the properties of metallic elements occurring in other parts of the periodic table. The actinide metals are often thought to be exotic, because they tend to have properties that are difficult to explain by simple theoretical approaches that have been useful for simple metals. The properties of the actinide metals do in fact represent a severe test to the theoretical solid-state scientist, as do the other transition-metal series. But, like other metals, they are lustrous and may be malleable they have, among their several crystalline structures, some simple atomic arrangements and they have relatively low electrical resistivities and high thermal conductivities. 

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

As many physical properties of the actinide metals depend significantly on the sample purity, refining of the metals is mandatory. The choice of the refining methods is determined by the chemical reactivity of the actinide metal in the presence of the constituents of air, by high temperature reactions with crucible materials, by the specific radioactivity and the availability of the actinide elements. 

Solid Compounds. Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 4. 

The electron configurations of the actinides in the gas phase are listed in Table 14.3. Whereas in the case of the lanthanides only up to two f electrons are available for chemical bonding, in the case of the actinides more than two f electrons may be engaged in chemical bonds (e.g. all the electrons in compounds of FT(VI) and Np(VII)). This is due to the relatively low differences in the energy levels of the 5f and 6d electrons up to Z 95 (Am). However, these differences increase with Z and the chemistry of elements with Z > 96 becomes similar to that of the lanthanides. The special properties of the actinides are evident from their oxidation states, plotted in Fig. 14.12 as a function of the atomic number. In contrast to the lanthanides, a tendency to form lower oxidation states is observed with the heavier actinides. The... 

Relativistic effects on the valence electrons are already evident by comparing the electropositive character of Fr and Ra with that of their preceding homologues. The ionization potentials of both elements are not lower than those of their homologues Cs and Ba, respectively, as expected by extrapolation, but the ionization potential of Fr is about the same as that of Cs and the ionization potential of Ra is somewhat higher than that of Ba. The influence of relativistic effects on the properties of the actinides is evident also from the tendency of the heavier actinides to form lower oxidation states. For example, Es already prefers the oxidation state Es2+. 

Similarly to the lanthanides, actinides in the elemental state are reactive electropositive metals and pyrophoric in finely dispersed form. Strong reducing agents are necessary to prepare the metals from their compounds, for instance reduction of the halides by Ca or Ba at 1200°C (e.g. Pup4 + 2Ca Pu -I- 2CaF2). Some properties of the actinides in the metallic state are fisted in Table 14.5. The number of metallic modifications and the densities are remarkably high for U, Np and Pu. Some modifications of these elements are of low symmetry this is an exception for metals that is explained by the influence of the f electrons. The properties of Am and the following elements correspond to those of the lanthanides. 

One probably can predict some of the crystallographic properties, of the tetrapositive element 104 by extrapolation from those of its homologs zirconium and hafnium. The ionic radii of tetrapositive zirconium (0.74 A) and hafnium (0.75 A) suggest an ionic radius of about 0.78 A for tetrapositive element 104, allowing for the smaller actinide rather than lanthanide contraction. Further one would expect the hydrolytic properties of element 104 and the solubilities of its compounds (such as the fluoride) to be similar to those of hafnium. The sum of... 

Properties of the Transition Elements Other Transition Elements A Variety of Uses Lanthanides and Actinides The Inner Transition Elements MiniLab 8.2 The Ion Gharges of a Transition Element... 

These elements are characterized by the fact that the differentiating electrons lie deeply buried in the extranuclear structure in the third highest energy shell (the N shell in the lanthanides and the O shell in the actinides) in consequence the elements in each series are closely similar in their chemical properties. The lanthanide metals are of very limited technical importance, and that of the actinide elements is primarily concerned with the part they play in nuclear reactions. We... 

The use of the actinide elements fall into three categories (i) for imderstanding fundamental chemistry and the nature of the periodic system, (ii) as products, in the large scale use of nuclear energy, and (iii) miscellaneous applications, where the particular physical, chemical or nuclear properties are valuable. Only the last aspect is discussed here, the others are treated elsewhere in this book. The availability of transuranium element isotopes suitable for experiments is listed in Table 16.4. 

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