Nobelium oxidation states

Since nobelium has an oxidation state of +3, its ions are capable of forming compounds with a few nonmetals, as follows ... 

In aqueous solution nobelium ions are most stable in the 2 oxidation state. In this oxidation state nobelium has a filled f-electron shell, 5f ", which is likely a major factor for its stability. The potential for the No(III)/No(II) couple has been calculated by Nugent et al. as 1.45 0.05 V [177]. A value of —1.4 to —1.5 V was determined by Silva and coauthors from experimental measurements [180]. David et al. have performed electrochemical amalgamation experiments for the reduction of No(II) to No(0) in aqueous acetate and citrate solutions [181]. They determined half-wave potentials of—1.709 0.006 V versus SCE in acetate and —1.780 0.004 V versus SCE in citrate. Their data was consistent with a reversible two-electron reduction process for which the data in acetate solution was taken as representative of a noncomplexing medium. The 1/2 value in acetate was converted to a value of —1.47 0.01 V versus SHE and subsequently used to derive a standard potential value of —2.49 0.06 V for the No(II)/No(0) couple. 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

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... 

In 1961, the Ghiorso group at Berkeley bombarded 3 micrograms of californium 251 with "B ions and obtained element 103 with an apparent atomic mass of 257 and ti/, of 8 seconds (later corrected to mass 258, ti/ 3.9 seconds). In 1965, the Joint Institute for Nuclear Reactions 0INR) in Dubna reported an isotope of mass 256 with ti/, of 35 seconds. In 1968, Ghiorso s group demonstrated that element 103 is in the +3 oxidation state as predicted by Seaborg, unlike its predecessor nobelium (which favors +2) but consistent with most of the other actinides. The new element was eventually named lawrencium (Lr) after Ernest O. Lawrence (1901-58), the inventor of the cyclotron. The most stable isotope is now known to be Lr (ti/ 3.6 hrs). 

Fm, the daughter of the new element, was collected using the recoil technique, one atom at a time, and identified as Fm by their position in the cation-exchange elution curve. A half-life of 3 s was assigned to No at that time. However, it is now known that the 3 s radioactivity was No produced in the Cm ( C,4n) reaction the used target contained 20 times more Cm than Cm. No is now known to have 55 s half-life. The errors in this experiment indicate the difficulty associated with one-atom-at-a-time studies. In subsequent chemical experiments, it was found that the most stable oxidation state of element 102 in solution was 2+. The element was named nobelium after Alfred Nobel. 

The existence of Md" " has been reported by Mikheev et al. (1973) but studies by other groups have failed to confirm these observations (Hulet et al. 1980). Both No and Md form readily in solution. No " is the most stable oxidation state of nobelium, a result of the stability of the completely filled 5f shell. Pulse radiolysis of acid solutions has been used to make unstable Am, Cm, Bk, Cf and Es. Fm is more stable than these ions but less stable than Fm. ... 

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. 

Laboratory. The isotope produced was the 20-hour Fm. During 1953 and early 1954, while discovery of elements 99 and 100 was withheld from publication for security reasons, a group from the Nobel Institute of Physics in Stockholm bombarded with O ions, and isolated a 30-min a-emitter, which they ascribed to 100, without claiming discovery of the element. This isotope has since been identified positively, and the 30-min half-life confirmed. The chemical properties of fermium have been studied solely with tracer amounts, and in normal aqueous media only the (III) oxidation state appears to exist. The isotope and heavier isotopes can be produced by intense neutron irradiation of lower elements such as plutonium by a process of successive neutron capture interspersed with beta decays until these mass numbers and atomic numbers are reached. Twenty isotopes and isomers of fermium are known to exist. Fm, with a half-life of about 100.5 days, is the longest lived. °Fm, with a half-life of 30 min, has been shown to be a product of decay of Element 102. It was by chemical identification of Fm that production of Element 102 (nobelium) was confirmed. Fermium would probably have chemical properties resembling erbium. 

Though the 3 + oxidation state remains a dominant feature of the heavier actinides, a tendency toward the formation of lower oxidation states has emerged. Divalency has been observed in solution for californium through nobelium. The 2+ oxidation state is much more stable in the heavy actinides than in the corresponding lanthanides as the end of the series is approached and, in fact, the increased M -< ctron binding makes the 2 + oxulation state the most stable in aqueous solution for nobelium. However, lawrendum, the last member of the actinide series, returns to the 3 + state as the most stable in solution, as expected. 

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