Characteristics of the Actinides

Ionized UCI4 was separated using a cyclotron-like system to obtain the first enriched samples used in the Manhattan project. In practice, this was a difficult method to utilize and was rejected in favour of gaseous diffusion. 

One process has involved selectively ionizing in uranium vapour using a tuneable laser. 

Early in the actinide series, electrons in the 6d orbitals are lower in energy than there is 5f orbitals, This is clear from the ground-state electronic configurations (Table 9.3) of the atoms, which show that the 6d orbitals are filled before 5f. The 5f orbitals are starting to be filled at protoactinium, and with the exception of curium, the fid orbitals are not occupied again. 

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Experiments seem to show that the element possesses a moderately stable dipositive (11) oxidation state in addition to the tripositive (111) oxidation state, which is characteristic of the actinide elements. 

The outstanding characteristic of the actinide elements is that their nuclei decay at a measurable rate into simpler fragments. Let us examine the general problem of nuclear stability. In Chapter 6 we mentioned that nuclei are made up of protons and neutrons, and that each type of nucleus can be described by two numbers its atomic number (the number of protons), and its mass number (the sum of the number of neutrons and protons). A certain type of nucleus is represented by the chemical symbol of the element, with the atomic number written at its lower left and the mass number written at its upper left. Thus the symbol... 

It should be noted that one of the most significant characteristics of the actinides is their radioactivity all isotopes are radioactive, although some have half-lives of greater than 1 x 10 years. Precautions must be taken in their handling, ranging from the use of special enclosures (HEPA-filtered exhaust hoods, negative-pressure gloveboxes) to the use of shielded facilities. 

Similarities exist between the chemical characteristics of the actinides and those of the lanthanides. The metal ions are generally considered to be relatively hard Lewis acids, susceptible to complexation by hard (i.e., first row donor atom) ligands and to hydrolysis. Both actinide and lanthanide ions are affected by the lanthanide contraction, resulting in a contraction of ionic radius and an increasing reluctance to exhibit higher oxidation states later in the series. Most species are paramagnetic, although the electron spin-nuclear spin relaxation times often permit observation of NMR spectra, and disfavor observation of ESR spectra except at low temperatures. The elements display more than one accessible oxidation state, and one-electron redox chemistry is common. 

Plant uptake of actinide elements varies greatly. Concentration ratios appear to be related to source characteristics, to chemical characteristics of the actinide in environmental... 

Uptake of Am, Cm, and Np by plants exceeds that observed for Pu differential uptake 1s postulated to be related to hydrolysis and sorption characteristics of the actinides. The order of uptake for these actinides (Np>Am2CmMJ>Pu) appears to be related to the order of oxidation state species, V>III VI>IV. 

A full discussion of the deposition and lung clearance characteristics of the actinides is beyond the scope of this chapter, but the subject area has been comprehensively and clearly reviewed in ICRP publication no. 48 (1986). It is germane, however, to consider some of the lung clearance characteristics determined by the initial physicochemical form of the actinide as this is vital to the targeting of subsequent chelation therapy and other modes of intervention. 

A summary of qualitative information about the oxidation-reduction characteristics of the actinide ions is presented in Table 14.6. The disproportionation and redox reactions of UO2, Pu, PuO, and Am02 are especially complex, and, despite extensive study, many aspects of these reactions still remain to be explored. In the case of plutonium, the situation is especially complicated, for ions in all four oxidation states iii, iv, v, and vi can exist simultaneously in aqueous solution in equilibrium with each other in comparable concentrations. The kinetics of the redox reactions of the actinide elements have been ably summarized by Newton [22]. 

Another characteristic of the actinide series is the fact that the L values for f go through a maximum not at the half-filled shell, as in the case of the S values, but at the one-fourth- and three-fourths-filled shell. Thus the ground state of Np I f ds is for Np I f d s it is These high J values are poorly excited... 

Purification methods vary in complexity with the chemical and physical characteristics of the actinide metal and with the quantity of material being processed. The principal refining processes, and their applicability, can be summarized as follows ... 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

In what follows we briefly review some of the previous attempts to analyze the available spectra of plutonium (6). In addition, we estimate energy level parameters that identify at least the gross features characteristic of the spectra of plutonium in various valence states in the lower energy range where in most cases, several isolated absorption bands can be discerned. The method used was based on our interpretation of trivalent actinide and lanthanide spectra, and the generalized model referred to earlier in the discussion of free-ion spectra. 

Actinium is the last (bottom) member of group 3 (IIIB) of elements in the periodic table and the first of the actinide series of metallic elements that share similar chemical and physical characteristics. Actinium is also closely related in its characteristics to the element lanthanum, which is located just above it in group 3. The elements in this series range from atomic number 89 (actinium) through 103 (lawrencium). Actiniums most stable isotope is actinium-227, with a half-life of about 22 years. It decays into Fr-223 by alpha decay and Th-227 through beta decay, and both of these isotopes are decay products from uranium-235. 

AH the isotopes of americium belonging to the transuranic subseries of the actinide series are radioactive and are artificially produced. Americium has similar chemical and physical characteristics and is hofflologous to europium, located just above it in the rare-earth (lanthanide) series on the periodic table. It is a bright-white malleable heavy metal that is somewhat similar to lead. Americiums melting point is 1,176°C, its boiling point is 2,607°C, and its density is 13.68g/cm. ... 

The characteristic structure of the actinide spectra is mainly determined by the special properties of the 5 f wave functions these, in turn, arise from the special nature of the 5 f effective potential-energy function Ueff (5f). Uetf describes the effect on the 5 f electrons of the attraction by the nucleus as well as of all the interactions with other electrons. It contains also a centrifugal term, 1(1 -I- l)/2 r, of particular importance for the properties of 5f wave functions, the effect of which will be illustrated later in more detail. 

The adaptation for radioactive samples described in Fig. If has been performed by J. Naegele basically for the investigation of surface and bulk properties of actinide sohds such as discussed in Chap. E. In that chapter, a more detailed description for the characteristics of the system is given... 

It is worthwhile to mention the ample use of screening final states models in understanding core levels as well as valence band spectra of the oxides. The two-hole models, for instance, which have been described here, are certainly of relevance. Interpretational difference exists, for instance, on the attribution of the 10 eV valence band peak (encountered in other actinide dioxides as well), whether due to the non-screened 5f final state, or to a 2p-type characteristics of the ligand, or simply to surface stoichiometry effects. Although resonance experiments seem to exclude the first interpretation, it remains a question as to what extent a resonance behaviour other than expected within an atomic picture is exhibited by a 5 f contribution in the valence band region, and to what extent a possible d contribution may modify it. In fact, it has been shown that, for less localized states (as, e.g., the 3d states in transition metals) the resonant enhancement of the response is less pronounced than expected. 

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. 

The differences in actinide element concentrations in the second vessel outlet samples for experiments with and without the presence of a gamma field (experiment 3 vs. experiments 1 and 2) are clearly evident from the data of Table VI. These differences are probably related to differing leaching characteristics of the waste with and without gamma radiation. Such effects have been reported by others (14, 15). We are planning experiments to verify and to further study this result. 

Cp3MR (M = U, Th, Np) CO Cp M X R Migratory insertion reaction. The r 2-bonding of the CO insertion products is characteristic of such actinide compounds 281, 421... 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

All the elements of the actinide series from Th to Cm form fluorite-type dioxides, and the series affords an opportunity of studying the nonstoichiometry and ordered defect phases characteristic of the structure. To date, the most complete set of results refers to uranium dioxide, and these are discussed first. 

The T -bonding of the CO and RNC insertion products is characteristic of these actinide compounds, but has some parallels in the organometallic chemistry of the d-block elements. There are similar insertions into An—NR2 bonds to give products with rf ligands, namely,... 

The absorption spectrum of the hexagonal form of BkCls as a function of time. The changes in the spectrum are due to the formation of CfCF. Note the sharp Tanthanide-like transitions characteristic of the later actinide (-1-3) state, (from J.R. Peterson et al, Inorg. Chem., 1986, 25, 3779 reproduced by permission of the American Chemical Society). 

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