The Actinide Elements

Interest in the actinides stems from the importance of measuring the physical nuclear constants required for the study of the nuclear structure in this region of comparative instability. Few low-energy levels are conveniently populated by a radioactive parent, but the successful and detailed studies with Np using both and Am parents have stimulated the use of more difficult techniques such as Coulomb excitation. Consequently the 2 Th, Pa, U, and Am resonances have now also been detected. Full details of the known nuclear parameters of these nuclides are tabulated in Appendix I. 

The 49-8-keV resonance of thorium-232 was reported in 1968 following experiments using Coulomb excitation of a thorium metal target by 4-5-MeV a-particles at 80 K [1]. The transition is highly internally converted (aj = 260), which reduces the effective excitation rate considerably. The linewidth obtained from an absorber of Th02 at 110 K was 16-7 mm s S which is close to the natural width of 15-7 mm s thereby implying negligible quadru-pole interaction, consistent with the cubic lattices of the metal and ThOa. No hyperfine effects have yet been reported. 

The 84-2-keV resonance of Pa was first reported in 1968 and also requires complicated experimentation [2]. The 25 5-hour precursor of Th is troublesome to prepare and has to be separated from impurities and fission products following an ( , y) reaction on separated °Th. The decay scheme is complex and the relevant details are shown in simplified form in Fig. 18.1. The 84-2-keV level is probably the third excited state of Pa. This isotope is itself radioactive and decays by a-emission with a half-life of 3-25 x 10 y. It is only recently that quantities of this isotope adequate for preparing chemical compounds have become available. 

Spectra with a source of ThOa and absorbers of PajOj and PaOi at 4-2 K are shown in Fig. 18.2. Although there is some suggestion of hyperfine interactions, the lines are far broader than the natural width of 0-079 mm s , which makes analysis difficult. If the hyperfine interactions are large compared to the linewidth, as appears likely, the effects of radiation damage on the environment of the Mossbauer atoms could be a significant factor. 

The 44-7-keV transition in suffers from an extremely high internal conversion coefficient oct of about 625. A resonance was first described in 1967 following Coulomb excitation of a metal target at 80 K by 3-MeV 

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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 actinide elements are a group of chemically similar elements with atomic numbers 89 through 103 and their names, symbols, atomic numbers, and discoverers are given in Table 1 (1-3) (see Thorium and thorium compounds Uranium and uranium compounds Plutonium and plutonium compounds Nuclear reactors and Radioisotopes). 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Isotopes sufficiently long-Hved for work in weighable amounts are obtainable, at least in principle, for all of the actinide elements through fermium (100) these isotopes with their half-Hves are Hsted in Table 2 (4). Not all of these are available as individual isotopes. It appears that it will always be necessary to study the elements above fermium by means of the tracer technique (except for some very special experiments) because only isotopes with short half-Hves are known. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

The close chemical lesemblance among many of the actinide elements permits their chemistry to be described for the most part in a correlative way... 

The actinide elements exhibit uniformity in ionic types. In acidic aqueous solution, there are four types of cations, and these and their colors are hsted in Table 5 (12—14,17). The open spaces indicate that the corresponding oxidation states do not exist in aqueous solution. The wide variety of colors exhibited by actinide ions is characteristic of transition series of elements. In general, protactinium(V) polymerizes and precipitates readily in aqueous solution and it seems unlikely that ionic forms ate present in such solutions. 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

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. 

F. L. Getting, M. H. Rand, and R. J. Ackermaim, ia F. L. Oettiag, ed.. The Chemical Thermodynamics of Actinide Elements and Compounds, Part 1, The Actinide Elements, SHlPDBj424j 1, IAEA, Vienna, Austria, 1976. 

Every known isotope of the actinide elements is radioactive and the half-lives are such that only possibly Pu could... 

Table 31.4 Oxidation states and stereochemistries of compounds of the actinides An is used as a general symbol for the actinide elements...

Table 31.5 Oxides of the Actinide Elements The most stable oxide of each element is printed in bold.

One name, more than any other, is associated with the actinide elements Glenn Seaborg (1912-1999). Between 1940 and 1957. Seaborg and his team at the University of California, Berkeley, prepared nine of these elements (at no. 94-102) for the first time. Moreover, in 1945 Seaborg made the revolutionary suggestion that the actinides, like the lanthanides, were filling an f sublevel. For these accomplishments, he received the 1951 Nobel Prize in chemistry. 

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... 

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