Curium oxidation states

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. 

Its stability then decreases progressively until we reach curium where aqueous solutions containing the tetra-positive state must be complexed by ligands such as fluoride or phosphotungstate. Even then, they oxidize water and revert to cur-ium(lll). The expected drop in I4 between curium and berkelium provides Bk" (aq) with a stability similar to that of Ce (aq), but the decrease in stability is then renewed, and beyond californium, the +4 oxidation state has not yet been prepared [2, 10, 15]. 

Curium is a synthetic (not natural) transuranic element of the actinide series. It was determined that curiums major valence and oxidation state was +3, similar to other elements of this series. The most stable isotope of curium is curium-247, with a half-life of 1.56xl0 years. 

The Mars Pathfinder rover carried an Alpha Proton X-ray Spectrometer (APXS), and the two Mars Exploration Rovers (MER - Spirit and Opportunity) carried Alpha Particle X-ray Spectrometers (also called APXS, but in this case more precise versions of the Pathfinder instrument, though without the ability to monitor protons for light element analyses). These instruments contained radioactive curium sources (Fig. 13.16) whose decay produced a-particles, which irradiated target rocks and soils. The resulting characteristic X-rays provided measurements of major and minor element abundances. The MER rovers also carried Mossbauer spectrometers, which yielded information on iron oxidation state. 

However, bearing in mind these caveats, we can make certain generalizations about the behavior of the actinide elements in natural waters. Americium and curium remain in the +3 oxidation state over the natural range of environmental conditions. For plutonium, Pu(III) is unstable to oxidation at environmental acidities, and so the other three states are observed with the dominant oxidation state in natural waters being Pu(V). [Humic materials cause a slow reduction of... 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

Since the uranyl ion is so obviously its own category, it is very interesting to compare with the analogous species formed by transuranium elements. M = Np, Pu and Am form all three MO 72 and MO which are, by no means, the most stable oxidation states of their elements, and which tend toward reduction by the radiochemical products concomitant with the high specific radioactivity of the isotopes normally studied of plutonium and americium (whereas e.g. 244Pu with the half-life 82 million years would not present this problem). Contrary to some reports in literature, it does not seem that curium (and the subsequent elements) form such dioxo complexes. 

Polarographic studies gave no evidence for the existence of the bivalent oxidation states of selected actinides in acetonitrile solution. Only one wave corresponding to reduction of americium(iii) or curium(iii) to the zero-valent state was observed and experiments with berkelium(iii) and einsteinium(iii) failed to give conclusive results because of rapid radiolysis of the acetonitrile solution. A study of the electrochemical reduction of americium, thulium, erbium, samarium, and europium showed that the elements did assume the bivalent state with the actinide bivalent cations having a smaller stability than the lanthanides. The half-wave potential of nobelium was found to be —1.6 V versus the standard hydrogen electrode for the reaction... 

Curium halides. Like americium, curium forms binary halides in the oxidation states of III and... 

For these elements, the correspondence of the actinide and the lanthanide series becomes most clearly revealed. The position of curium corresponds to that of gadolinium where the / shell is half-filled. For curium, the +3 oxidation state is the normal state in solution, although, unlike gadolinium, a solid tetrafluoride, CmF4, has been obtained. Berkelium has +3 and +4 oxidation states, as would be expected from its position relative to terbium, but the +4 state of terbium does not exist in solution whereas for Bk it does. 

Americium in aqueous solution is in the -1-3, -1-5, and -1-6 states, respectively, as Am+, AmO and AmOj. The trivalent state is most common, but higher oxidation states are achieved by strong oxidation. The highest oxidation states can be used for separating americium from curium and rare earth radionuclides with ammonium persulfate, (NH4)2S208 (Penneman and Keenan 1960). 

The superheavy elements are leading to deeper understanding of nuclear chemistry and the periodic table much as the lanthanides did at the start of the 20th century and the actinides did in mid-century. The favored +3 oxidation states of the lanthanides have electronic configurations 4f, 4f, 4f. .. 4f 4f 4f for the 14 consecutive lanthanides Ce through Lu respectively. These rare earths are chemically similar (and hard to separate) because their 4f orbitals exert little influence on reactivity. When Glenn T. Seaborg synthesized americium (95) and curium (96) in 1944, he discovered that, like the lanthanides, they readily form +3 oxidation states and fill 5f orbitals. In 1944, he proposed a new transi-... 

In the determination of transuranium elements (or nuclides), the most important step is separation of the elements from the sample matrix. Differences in redox properties are used for the separation of the first four elements in the series (neptunium, plutonium, americium, curium). Since the higher members exist primarily in the same oxidation state (III), separation by ion-exchange chromatography is commonly used. The lighter transuranic elements can be determined by common chemical methods, and trace amounts are usually determined by radiometric methods such as a-spectrometry. 

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