Actinides, oxidation numbers

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

In Fig. 1, we have plotted the oxidation numbers of the actinides and of the lanthanides. We see that for the lanthanides the valence 3 is the most stable valence throughout the series. There are exceptions Ce displays for instance tetravalency in many compounds Eu and Yb display divalency. These exceptions are understood e.g., Eu and Yb are at the half-filling and at the filling of the 4f shell, which are stable electronic configurations. There is a tendency for both to share just the two outer 5 s electrons in bonding, displaying therefore, divalency, and preserve these stable configurations. 

On the contrary, there is a spread of oxidation numbers for the light actinides (at least up to Cm), which, for Pu and Np, range from 3 to 7 After Cm, however, the trivalent oxidation state is always met, and this second half of the actinide series approaches more the behaviour of the lanthanides. 

Fig. 1. Oxidation numbers for - d-transition series , - f-transition series (lanthanides and actinides) (non-common or uncertain oxidation numbers have been put between brackets)...

The preparation and properties of numerous actinide haUdes have been described by D. Brown Although the oxidation numbers of actinides in halides can vary from II to VI, most solid state studies are limited to di-, tri- and tetrahalides. 

Ouster molecular ccdculations, a number of which has appeared on actinide oxides , are very sensitive to covalent mixing of the actinide-oxygen external orbitals which has a local bonding character. This is emphasized by these calculations which take into account a finite number of shells around the bonded species. It is just possible that the cluster method has a tendency to overestimate covalency. 

As mentioned in Section 17.3.1, retention of quadrivalent actinide oxides within the phosphate matrix is not a major issue because these oxides are insoluble in water, and all that is needed is their microencapsulation by the phosphate components of the matrix. This was demonstrated in a number of studies on UO2 and PUO2 and their surrogate Ce02. If the actinides are found in a trace amount in the waste, their chemical form is not so important because the phosphate matrix immobilizes them very efiectively. For example, the wastewater in the case study given in Section 16.3.2.2 contained 32 pCi/ml of and 0.6 pCi/ml of The ANS 16.1 tests conducted on the waste forms with 18.6pCi/g loading of combined U in the waste form showed that the leaching index was 14.52. XCLP tests also showed that levels in the leachate were below the detection limit of 0.2 pCi/ml. This implies that microencapsulation of trace-level U is very efiective in the Ceramicrete matrix. 

The rules above gave maximum and minimum oxidation numbers, but those might not be the only oxidation numbers or even the most important oxidation numbers for an element. Elements of the last six groups of the periodic table, for example, may have several oxidation numbers in their compounds, most of which vary from one another in steps of 2. For example, the major oxidation states of chlorine in its compounds are -1, +1, +3, +5, and +7. The transition metals have oxidation numbers that may vary from one another in steps of 1. The inner transition elements mostly form oxidation states of +3, but the first part of the actinide series acts more as transition elements and the elements have maximum oxidation numbers that increase from +4 for Th to +6 for U. These generalizations are not absolute rules, but allow students to make educated guesses about possible compound formation without exhaustive memorization. These possibilities are illustrated in Fig. 14-1. 

The thorium oxide system is dominated by Th02. The dioxide can be synthesized by burning a number of thorium compounds, including hydroxides, oxalates, carbonates, and so on. The Th02 crystalhzes in the cubic fluorite structure. Th02 is very heat resistant as are all of the actinide oxides and melts at 3390 °C, which is the highest for any known metal oxide. 

The maximum oxidation number of any atom in any of its compounds is equal to its periodic group number, with a few exceptions. The coinage metals have the following maximum oxidation numbers Cu, +2 Ag, +2 and Au, +3. Some of the noble gases (group 0) have positive oxidation numbers. Some lanthanide and actinide element oxidation numbers exceed 3, their nominal group number. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

The two rows beneath the main body of the periodic table are the lanthanides (atomic numbers 58 to 71) and the actinides (atomic numbers 90 to 103). These two series are called inner transition elements because their last electron occupies inner-level 4/orbitals in the sixth period and the 5/orbitals in the seventh period. As with the d-level transition elements, the energies of sublevels in the inner transition elements are so close that electrons can move back and forth between them. This results in variable oxidation numbers, but the most common oxidation number for all of these elements is 3+. 

The lanthanides and actinides, called the inner transition elements, occupy the/region of the periodic table. Their valence electrons are in s and /orbitals. Inner transition elements exhibit multiple oxidation numbers. 

The lanthanides and actinides react by losing the valence electrons in their s orbitals. Because these elements can also lose electrons from their d and /orbitals, they have multiple oxidation numbers. 

The oxidation number +4 is not known in aqueous solutions of americium and curium. The measurement of the absorption spectra of americium and curium tetrafluorides by Asprey and Keenan (7) is thus a valuable contribution to the study of the electronic spectra of the actinide elements. Special techniques were devised to measure these spectra with microgram amounts of CmF4 and AmF4 over the region 3500-20000 A. Table XIV lists the positions and the relative intensities of the observed maxima. The agreement of the trivalent fluoride spectra with those of the corresponding... 

Transformed rare earth and actinide intermetallic compounds are shown to be very active as catalysts for the synthesis of hydrocarbons from CO2 and hydrogen. Transformed LaNis and ThNis the most active of the materials studied they have a turnover number for CH formation of 2.7 and 4.7 X 10 sec at 205°C, respectively, compared with I X 10 sec for commercial silica-supported nickel catalysts. Nickel intermetallics and CeFe2 show high selectivity for CHj formation. ThFcs shows substantial formation of C2H6 (15%) as well as CHi,. The catalysts are transformed extensively during the experiment into transition metal supported on rare earth or actinide oxide. Those mixtures are much more active than supported catalysts formed by conventional wet chemical means. 

There is one way in which the actinide series definitely differs from the lanthanide series. This is in the existence of oxidation states higher than IV [protactinium (V), uranium(VI), neptunium (VI), plutonium (VI)] in this series. It must be concluded that the 5f electrons are not so tightly bound as the 4f electrons. This is certainly reasonable. However, the evidence so far is in favor of a maximum oxidation number of VI in this series, so that the removal of three electrons, or four if there are no electrons in the 6d orbitals, from the 5f orbitals is the maximum that occurs in ordinary chemical reactions. 

The synthesis of lanthanide and actinide compounds is the topic of a book edited by Meyer and Morss (1991). Topics that relate to halides, with the author(s) in brackets, include Lanthanide fluorides [B.G. Muller], Actinide fluorides [N.P. Freestone], Binary lanthanide(III) halides, RX3, X = Cl, Br, and I [G. Meyer], Complex lan-thanide(III) chlorides, bromides and iodides [G. Meyer], Conproportionation routes to reduced lanthanide halides [J.D. Corbett], and Action of alkali metals on lanthanide(III) halides an alternative to the conproportionation route to reduced lanthanide halides [G. Meyer and T. Schleid]. Meyer and Meyer (1992) reviewed lanthanide halides in which the valence of the lanthanide was considered unusual, with unusual being defined as compounds in which the localized valence of an atom differs from its oxidation number. A metallic halide such as Lalj [oxidation number (0)= -1-2 valence (V)= -l-3, since the 5d electron is delocalized in the conduction band] or a semiconducting halide such as PrjBtj (O = -t- 2.5 V = -I- 3) is unusual by this definition, but Tmlj (O = -1-2 V = +2) is not. In this review synthesis, properties, and calculated electronic structures are considered with emphasis on praseodymium halides and hydrogen intercalation into lanthanide dihalides and monohalides . 

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