Berkelium oxidation state

Although the berkelium oxidation states, 0 iii, and iv are known in bulk phase, further work is required to characterize more completely the solid-state and solution chemistries of this element. The synthesis of divalent berkelium in bulk should be possible via the metallothermic reduction of its trihalides, e.g. 

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]. 

The chemical properties of berkehum are rare earth-like character because of its half-filled 5/ subsheU and should be simdar to cerium. The element readily oxidizes to berkelium dioxide, Bk02 when heated to elevated temperatures (500°C). In aqueous solutions, the most common oxidation state is -i-3 which may undergo further oxidation to +4 state. A few compounds have been synthesized, the structures of which have been determined by x-ray diffraction methods. These include the dioxide, Bk02 sesquioxide, Bk203 fluoride,... 

Berkelium is known to exist 111 aqueous solution in two oxidation states, the (III) and the (IV) states, and the ionic species presumably correspond to Bk+3 and Bk+4. The oxidation potential for the berkelium(lll)-berkehum(IV) couple is about —1.6 V on the hydrogen scale (hydrogen-hydrogen ion couple taken as zero)... 

The solubility properties of berkelium in its two oxidation states are entirely analogous to those of Lite actmide and lanLlianide elements in the corresponding oxidation states, Thus in the tripositive state such compounds as the fluoride and the oxalate arc insoluble in add solution, and the tetrapositive slate has such insoluble compounds as the lodate and phosphate in acid solution. The nitrate, sulfate, halides, perchlorate, and sulfide of both oxidation states are soluble,... 

The references given in Table I are those describing the preparation of a given compound the reference may or may not contain information on the behavior of the compound with time Note that the compounds have been synthesized in different oxidation states and different crystal structures where possible Not shown in the table are einsteinium, berkelium, and californium phosphates which have also been prepared and are being studied at present (11) ... 

Bulk-Phase Compounds Some of our results in the studies of the bulk-phase compounds have been published (3-7) These studies have shown that oxidation state is preserved for these actinides in either a or fT decay Trivalent einsteinium will transmute to trivalent berkelium which transmutes to trivalent californium It has also been observed that divalent einsteinium yields divalent californium. It is interesting to note in this latter case that it has not yet been possible to synthesize divalent berkelium in the bulk phase Berkelium(II) has not been observed in our aged einsteinium(II) compounds either, but it would be logical to assume it has been produced there. Our inability to observe Bk(II) could be related to weak absorption intensities and/or interference by absorption bands of einsteinium(II) or... 

Berkelium exhibits both the III and IV oxidation states, as would be expected from the oxidation states displayed by its lanthanide counterpart, terbium. Bk(III) is the most stable oxidation state in noncomplex-ing aqueous solution. Bk(IV) is reasonably stable in solution, undoubtedly because of the stabilizing influence of the half-filled Sf7 electronic configuration. Bk(III) and Bk(IV) exist in aqueous solution as the simple hydrated ions Bk3+(aq) and Bk4+(aq), respectively, unless com-plexed by ligands. Bk(III) is green in most mineral acid solutions. Bk(IV) is yellow in HC1 solution and is orange-yellow in H2S04 solution. A discussion of the absorption spectra of berkelium ions in solution can be found in Section IV,C. 

The preparation and characterization of intermetallic compounds and alloys of berkelium should be pursued, as well as the determination of the stability constants of Bk(IV) complexes. The range of oxidation states accessible to berkelium might be expanded by stabilizing Bk(II) and/or Bk(V) in highly complexing aqueous, nonaqueous, or even molten salt media and/or in appropriate solid-state matrices. 

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... 

This is the only important oxidation state for Th, and is one of the two for which U is stable in aqueous solution it is moderately stable for Pa and Np also. In water Pu, like Pu, dispropor-tionates into a mixture of oxidation states III, IV, V and VI, while Am not only disproportionates into Am -I- Am 02" " but also (like the strongly oxidizing Cm ) undergoes rapid self-reduction due to its a-radioactivity. As a result, aqueous Am and Cm require stabilization with high concentrations of F ion. Berkelium(IV), though easily reduced, clearly has an enhanced stability, presumably due to its f configuration, and the only other -1-4 ion is Cf, found in the solids CfFq and Cf02. 

Hum have mass numbers that range from 240 to 251, and are all radioactive. The longest-lived isotope has a mass number of 247 and a half-life of 1,380 years. The ground state electronic configuration of the outer orbitals of berkelium is 5f 6cf7s. In compounds and in aqueous solution, berke-lium is present in oxidation states III (the more stable) and IV. 

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. 

To identify the new nuclide, a rapid cation-exchange separation technique using ammonium citrate as an eluant was employed. Early experiments indicated that element 97 had two oxidation states 3+ and 4+. The actinide concept provided the guidance to search for these two oxidation states, by analogy with the homolog element, terbium (Tb). The chemically separated samples were subjected to the measurement of radiation. Characteristic Cm X-rays associated with the electron capture (EC) decay and low-intensity a particles with a half-life of 4.5 h were detected. Berkelium was named after the city of Berkeley, California where it was discovered, just as the name terbium derived from Ytterby, Sweden. 

The electronic configurations 5f or 4f representing the half-filled f shells of curium and gadolinium, have special stability. Thus, tripositive curium and gadolinium, are especially stable. A consequence of this is that the next element in each case readily loses an extra electron through oxidation, so as to obtain the f structure, with the result that terbium and especially berkelium can be readily oxidized from the III to the IV oxidation state. Another manifestation of this is that europium (and to a lesser extent samarium) -just before gadolinium - tends to favor the 4f structure with a more stable than usual II oxidation state. Similarly, the stable f electronic configuration leads to a more stable than usual II oxidation state in ytterbium (and to a lesser extent in thuUum) just before lutetium (whose tripositive ion has the 4f structure). This leads to the prediction that element 102, the next to the last actinide element, will have an observable II oxidation state. 

With the exception of thorium and protactinium, all of the actinide elements show a -1-3 oxidation state in aqueous solution. A stable +A state is observed in the elements thorium through plutonium and in berkelium. The oxidation state -1-5 is well established for the elements protactinium through americium, and the -1-6 state is well established in the elements uranium through americium. The oxidation state +2 first appears at californium and becomes increasingly more stable in proceeding to nobelium. 

In solution, the range of oxidation states accessible to berkelium should be further examined by using strong complexing agents in an effort to stabilize Bk(ii), Bk(iv), and possibly Bk(v), produced chemically or electrochemically in non-aqueous or molten-salt media. New organometallic complexes of Bk(iii)... 

Berkelium is the first member of the second half of the actinide series of elements. Extended knowledge of the stability and accessibility of the various oxidation states of berkelium is important to the understanding and predictability of its physicochemical behavior. In addition, such information would enable more accurate extrapolations to the physicochemical behavior of the transberkelium elements for which experimental studies are severely limited by lack of material and/or by intense radioactivity. 

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