Nobelium

Nobelium is a synthetic chemical element; it has symbol No and atomic number 102. It is named after Alfred Nobel, the inventor of dynamite and benefactor of science. A radioactive metal, it is the tenth transuranium element, the second transfermium, and is the fourteenth member of the actinide series. Like all elements with atomic number over 100, nobelium can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of twelve nobelium isotopes are known to exist; the most stable is ²⁵⁹No with a half-life of 58 minutes, but the shorter-lived ²⁵⁵No (half-life 3.1 minutes) is most commonly used in chemistry because it can be produced on a larger scale.

Chemistry experiments have confirmed that nobelium behaves as a heavier homolog to ytterbium in the periodic table. The chemical properties of nobelium are not completely known: they are mostly only known in aqueous solution. Before nobelium's discovery, it was predicted that it would show a stable +2 oxidation state as well as the +3 state characteristic of the other actinides; these predictions were later confirmed, as the +2 state is much more stable than the +3 state in aqueous solution and it is difficult to keep nobelium in the +3 state.

In the 1950s and 1960s, many claims of the discovery of nobelium were made from laboratories in Sweden, the Soviet Union, and the United States. Although the Swedish scientists soon retracted their claims, the priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists. It was not until 1992 that the International Union of Pure and Applied Chemistry (IUPAC) credited the Soviet team with the discovery. Even so, nobelium, the Swedish proposal, was retained as the name of the element due to its long-standing use in the literature.

Introduction

Discovery

thumb|right|The element was named after [[Alfred Nobel]]

The discovery of element 102 was a complicated process and was claimed by groups from Sweden, the United Kingdom, the United States, and the Soviet Union. The first complete and incontrovertible report of its detection only came in 1966 from the Joint Institute of Nuclear Research at Dubna (then in the Soviet Union).

The first announcement of the discovery of element 102 was announced by physicists from Argonne National Laboratory, Harwell Atomic Energy Research Establishment, and Nobel Institute for Physics in Sweden in 1957. The team reported that they had bombarded a curium target with carbon-13 ions in half-hour intervals for fifty times. Between bombardments, ion-exchange chemistry was performed on the target. Twelve out of the fifty bombardments contained samples emitting (8.5 ± 0.1) MeV alpha particles, which were in drops which eluted earlier than fermium (atomic number Z = 100) and californium (Z = 98). The half-life reported was 10 minutes and was assigned to either ²⁵¹No or ²⁵³No, although the possibility that the alpha particles observed were from a presumably short-lived mendelevium (Z = 101) isotope created from the electron capture of element 102 was not excluded. which was immediately approved by IUPAC,

In 1958, scientists at the Lawrence Berkeley National Laboratory repeated the experiment. The Berkeley team, consisting of Albert Ghiorso, Glenn T. Seaborg, John R. Walton and Torbjørn Sikkeland, used the new heavy-ion linear accelerator (HILAC) to bombard a curium target (95% ²⁴⁴Cm and 5% ²⁴⁶Cm) with ¹³C and ¹²C ions. They were unable to confirm the 8.5 MeV activity claimed by the Swedes but were instead able to detect decays from ²⁵⁰Fm, supposedly the daughter of ²⁵⁴No (produced from the ²⁴⁶Cm), which had an apparent half-life of ~3 s. Probably this assignment was also wrong, as later 1963 Dubna work showed that the half-life of ²⁵⁴No is significantly longer (about 50 s). It is more likely that the observed alpha decays did not come from element 102, but rather from ^250mFm.

In 1959, the team continued their studies and claimed that they were able to produce an isotope that decayed predominantly by emission of an 8.3 MeV alpha particle, with a half-life of 3 s with an associated 30% spontaneous fission branch. The activity was initially assigned to ²⁵⁴No but later changed to ²⁵²No. However, they also noted that it was not certain that element 102 had been produced due to difficult conditions.

In 1992, the IUPAC-IUPAP Transfermium Working Group (TWG) reassessed the claims of discovery and concluded that only the Dubna work from 1966 correctly detected and assigned decays to nuclei with atomic number 102 at the time. The Dubna team are therefore officially recognized as the discoverers of nobelium, although it is possible that it was detected at Berkeley in 1959. Because of outcry over the 1994 names, which mostly did not respect the choices of the discoverers, a comment period ensued, and in 1995 IUPAC named element 102 flerovium (Fl) as part of a new proposal, after either Georgy Flyorov or his eponymous Flerov Laboratory of Nuclear Reactions. This proposal was also not accepted, and in 1997 the name nobelium was restored.

Characteristics

Physical

thumb|upright=1.6|right|Energy required to promote an f electron to the d subshell for the f-block lanthanides and actinides. Above around 210 kJ/mol, this energy is too high to be provided for by the greater [[crystal energy of the trivalent state and thus einsteinium, fermium, and mendelevium form divalent metals like the lanthanides europium and ytterbium. Nobelium is also expected to form a divalent metal, but this has not yet been confirmed.]]

In the periodic table, nobelium is located to the right of the actinide mendelevium, to the left of the actinide lawrencium, and below the lanthanide ytterbium. Nobelium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible. Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties. The conclusion was that the increased binding energy of the [Rn]5f¹³6d¹7s² configuration over the [Rn]5f¹⁴7s² configuration for nobelium was not enough to compensate for the energy needed to promote one 5f electron to 6d, as is true also for the very late actinides: thus einsteinium, fermium, mendelevium, and nobelium were expected to be divalent metals, although for nobelium this prediction has not yet been confirmed. In 1986, nobelium metal was estimated to have an enthalpy of sublimation between 126 kJ/mol, a value close to the values for einsteinium, fermium, and mendelevium and supporting the theory that nobelium would form a divalent metal. Its density is predicted to be around 9.9 ± 0.4 g/cm³. It was largely expected before the discovery of nobelium that in solution, it would behave like the other actinides, with the trivalent state being predominant; however, Seaborg predicted in 1949 that the +2 state would also be relatively stable for nobelium, as the No²⁺ ion would have the ground-state electron configuration [Rn]5f¹⁴, including the stable filled 5f¹⁴ shell. It took nineteen years before this prediction was confirmed.

In 1967, experiments were conducted to compare nobelium's chemical behavior to that of terbium, californium, and fermium. All four elements were reacted with chlorine and the resulting chlorides were deposited along a tube, along which they were carried by a gas. It was found that the nobelium chloride produced was strongly adsorbed on solid surfaces, proving that it was not very volatile, like the chlorides of the other three investigated elements. However, both NoCl₂ and NoCl₃ were expected to exhibit nonvolatile behavior and hence this experiment was inconclusive as to what the preferred oxidation state of nobelium was.

It is expected that the relativistic stabilization of the 7s subshell greatly destabilizes nobelium dihydride, NoH₂, and relativistic stabilisation of the 7p_1/2 spinor over the 6d_3/2 spinor mean that excited states in nobelium atoms have 7s and 7p contribution instead of the expected 6d contribution. The long No–H distances in the NoH₂ molecule and the significant charge transfer lead to extreme ionicity with a dipole moment of 5.94 D for this molecule. In this molecule, nobelium is expected to exhibit main-group-like behavior, specifically acting like an alkaline earth metal with its ns² valence shell configuration and core-like 5f orbitals.

Nobelium's complexing ability with chloride ions is most similar to that of barium, which complexes rather weakly. The positive value shows that No²⁺ is more stable than No³⁺ and that No³⁺ is a good oxidizing agent. While the quoted values for the E°(No²⁺→No⁰) and E°(No³⁺→No⁰) vary among sources, the accepted standard estimates are −2.61 and −1.26 V. this value was later refined to be 6.62621 eV (639.33 kJ/mol). The ionic radius of hexacoordinate and octacoordinate No³⁺ had been preliminarily estimated in 1978 to be around 90 and 102 pm respectively; Of these, the longest-lived isotope is ²⁵⁹No with a half-life of 58 minutes, and the longest-lived isomer is ^251mNo with a half-life of 1.02 seconds. However, the still undiscovered isotope ²⁶¹No is predicted to have a still longer half-life of 3 hours. Additionally, the shorter-lived ²⁵⁵No (half-life 3.52 minutes) is more often used in chemical experimentation because it can be produced in larger quantities from irradiation of californium-249 with carbon-12 ions.

The half-lives of nobelium isotopes increase smoothly from ²⁵⁰No to ²⁵³No. However, a dip appears at ²⁵⁴No, and beyond this the half-lives of even-even nobelium isotopes drop sharply as spontaneous fission becomes the dominant decay mode. For example, the half-life of ²⁵⁶No is almost three seconds, but that of ²⁵⁸No is only 1.2 milliseconds. This shows that at nobelium, the mutual repulsion of protons poses a limit to the region of long-lived nuclei in the actinide series. The even-odd nobelium isotopes mostly continue to have longer half-lives as their mass numbers increase, with a dip in the trend at ²⁵⁷No. The thin layer of nobelium collected on the foil can then be removed with dilute acid without completely dissolving the foil.