Curium elements

As Z increases, the overall yield of the target element falls sharply, because many steps are required. Plutonium (element 94) has been produced in ton quantities by neutron bombardment of uranium-238. Up to curium (element 96), production in kilogram quantities is possible, but the yields fall by about one order of magnitude for each successive element beyond Z = 96. 

Americium (element 95) is prepared by Seaborg, R. A. James, L. O. Morgan, and A. Chiorso curium (element 90) by Seaborg, James, and A. Ghiorso. 

Chemistry of the Transuranium Elements (Verlag Chemie. Weinheim, English Ed., 1971) pp 619-622 Silva, "Trans-Curium Elements" in MTP Int. Rev. Set Inorg. Chem., Ser. One vol. 8, A. G. Maddock, Ed. (University Park Press, Baltimore, 1972) pp 71-105 Ghiorso, Handb. Exp. Phar-makol 36, 691-715 (1973). 

Keller, C., Angew. Chem. Internat. Edn., 1965, 4, 903 (synthesis of trans-curium elements by heavy-ion bombardments). 

After their success with neptunium and plutonium, Seaborg and his collaborators continued to look for more transuranics. (These collaborators included other scientists and graduate students who contributed many ideas and most of the work, and we regret that they must be consigned to a footnote.) They used cyclotron bombardment, a variety of targets, and microchemical techniques developed by Hahn. Americium and curium (elements number 95 and 96) were discovered in wartime at the Metallurgical Laboratory of the University of Chicago,... 

As mentioned above, a very important point is the presence of seven 5f electrons in stable tripositive curium (element 96), making this element very actinium-like. A series of thoride elements, e.g., would imply stable IV oxidation states in elements 95 and 96 and the presence of seven 5f electrons and the IV state almost exclusively in element 97. A series of this type seems to be ruled out by the now-known instability of americium in solution in the IV state and by the apparent non-existence in aqueous solution of any oxidation state other than III in curium. Moreover, the III state of uranium would be surprising on this basis, because this element would be the second member of a thoride, or IV oxidation state , series. The fact that nearly a year was spent in an unsuccessful effort to separate tracer amounts of americium and curium from the rare earths, immediately following the discovery of these two elements, illustrates how unnatural it would be to regard them as members of a thoride, or IV oxidation state, group. 

Curium, element 96, was also prepared by Seaborg s group at the University of Chicago. In 1944 they bombarded plutonium with helium ions and obtained the isotope Cm. 

Curium, element 96, is the element of highest atomic number that is available on the multigram scale. Even so, microchemical handling techniques are usually used [7,8]. The element, unknown in nature, is named after Pierre and Marie Curie, by analogy with its lanthanide congener, gadolinium (named after the Finnish chemist, J. Gadolin). The discovery of curium preceded that of americium (element 95). 

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

In the actinides, the element curium, Cm, is probably the one which has its inner sub-shell half-filled and in the great majority of its compounds curium is tripositive, whereas the preceding elements up to americium, exhibit many oxidation states, for example -1-2, -1-3. -1-4, -1-5 and + 6, and berkelium, after curium, exhibits states of -1- 3 and -E 4. Here then is another resemblance of the two series. 

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. 

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

Abb. 4. ce-Zerfallsenergien ( ) der Elemente Aktinium bis Curium in Abhangigkeit der Massenzahlen (A),... 

Tabelle 21 faBt die a-Halbwertszeiten und die spontanen Kem-spaltungshalbwertszeiten der geradzahligen Isotope der geradzahligen Elemente Plutonium (Z = 94) und Curium (Z = 96) zusammen. 

These elements have all been named for famous scientists or for the places of their creation. For example, americium, berkelium, and californium were named after obvious geographical locations. Nobelium was named for the Nobel Institute, although later study proved it was not really created there. Curium was named for Marie Curie, the discoverer of radium. Einsteinium was named for the famous physicist, Albert Einstein. Fermium and lawrencium were named for Enrico Fermi and Ernest O. Lawrence, who made important discoveries in the field of radioactivity. Mendelevium was named for the discoverer of the periodic chart. 

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