Properties of the Actinides

All actinides are radioelements and only Th and U have half-lives long enough to justify neglecting their radioactivity in some special chemical or technical operations. Ac and Pa are present in small amounts as decay products of U and Th (Table 11.3). Extremely small amounts of Np and Pu are produced in U by neutrons from cosmic radiation 10 1, Pu/ U 10 1. Harkin s rule is 

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 

Atomic number Symbol Name of the element Electron configuration 

With respect to the chemical properties of the actinides, a new effect becomes noticeable with increasing atomic number Z, the influence of the positive nuclear charge on the electrons increases in such a way that their velocity approaches the velocity of light, which leads to relativistic effects. The valence electrons are more effectively screened from the nuclear charge, with the result of stabilization of the spherical 7s and 7pi/2 orbitals and destabilization of the 6d and 5f orbitals. 

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+. 

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Some of the more important known properties of the actinides are summarized in Table 31.2. 

The interpretation of the electronic and especially of the magnetic properties of the actinides is much more complicated than in the lanthanides for the following reasons ... 

A critical assessment of the chemical thermodynamic properties of the actinides and their compounds is presently being prepared by an international team of scientists under the auspices of the International Atomic Energy Agency (Vienna). As a result of this effort, four publications (1, 2, 3, 5) have already become available and a further ten 6-T4), including the halides (8) and aqueous complexes with Tnorganic ligands (12),... 

Another difficulty arises from the chemical properties of the actinide metals. They are chemically reactive, rapidly corroded by moist air, pyrophoric, and, when in the molten state, dissolve common crucible materials. The radioactivity of short-lived isotopes of Am and Cm makes their long-term storage difficult small amounts can be stored successfully under ultrahigh vacuum. Large amounts of isotopes such 238pu with a Ti/2 of only 87.7 years are best stored under a pure inert... 

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. 

In this chapter our aim is to stress the main physical features governing the unique magnetic properties of the actinides and to illustrate these features by clear examples when available. 

The most important factor governing the physical, and in particular magnetic, properties of the actinides, is the extended nature of the 5f wave functions. Depending on ... 

In this chapter we are concerned with the magnetic properties of the actinides. How the localization of electrons belonging to an incomplete shell is related to their magnetic properties This is an old question to which it is possible to answer qualitatively if not quantitatively. There are 2 extreme points of view to approach this crucial problem,... 

At this point, the reader has been able to appreciate the richness and the variety of the magnetic properties of the actinide metals and compounds. The importance of magnetic measurements for the study of the actinides is thus clearly evidenced and will not decrease in the future. 

The properties of the actinides in the first half of the series are deduced from the characteristics of 5 f wavefunctions, in many ways similar to the d-wavefunctions in transition metals. 

Figure 11.2 Qualitative features of the fission barriers for actinide nuclei. (From H. C. Britt, Fission Properties of the Actinides in Actinides in Perspective, N. Edelstein, Ed. Copyright 1982 Pergamon Press, Ltd. Reprinted by permission of H. C. Britt.)...

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. 

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

Electronic/Magnetic Properties of the Actinides Further Reading... 

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. 

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. 

The spectra and chemical properties of the actinides vary greatly. For example, the spectra of U022+ and U1 are shown in Fig. 1. In the presence of fluoride, UO22 remains in solution, but UFi+ precipitates. Thus, combination of photochemical and thermal chemical properties can be used in their separations. Both oxidation and reduction have been reported in the literature as photochemical reactions of actinides. The reported reactions are summarized in Fig. 2. 

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