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=                              Nobelium                              =
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                            Introduction
======================================================================
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 penultimate
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.


                             Discovery
======================================================================
The discovery of element 102 was a complicated process and was claimed
by groups from Sweden, 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 at the Nobel Institute for Physics in Sweden in 1957.
The team reported that they had bombarded a curium target with
carbon-13 ions for twenty-five hours in half-hour intervals. 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 251102 or
253102, 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. The
team proposed the name 'nobelium' (No) for the new element, which was
immediately approved by IUPAC, a decision which the Dubna group
characterized in 1968 as being hasty.

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 fermium-250, supposedly the
daughter of 254102 (produced from the curium-246), 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 Swedish team attempted to explain the Berkeley team's
inability to detect element 102 in 1958, maintaining that they did
discover it. However, later work has shown that no nobelium isotopes
lighter than 259No (no heavier isotopes could have been produced in
the Swedish experiments) with a half-life over 3 minutes exist, and
that the Swedish team's results are most likely from thorium-225,
which has a half-life of 8 minutes and quickly undergoes triple alpha
decay to polonium-213, which has a decay energy of 8.53612 MeV. This
hypothesis is lent weight by the fact that thorium-225 can easily be
produced in the reaction used and would not be separated out by the
chemical methods used. Later work on nobelium also showed that the
divalent state is more stable than the trivalent one and hence that
the samples emitting the alpha particles could not have contained
nobelium, as the divalent nobelium would not have eluted with the
other trivalent actinides. Thus, the Swedish team later retracted
their claim and associated the activity to background effects.

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
254102 but later changed to 252102. However, they also noted that it
was not certain that element 102 had been produced due to difficult
conditions. The Berkeley team decided to adopt the proposed name of
the Swedish team, "nobelium", for the element.

: +  →  →  + 4

Meanwhile, in Dubna, experiments were carried out in 1958 and 1960
aiming to synthesize element 102 as well. The first 1958 experiment
bombarded plutonium-239 and -241 with oxygen-16 ions. Some alpha
decays with energies just over 8.5 MeV were observed, and they were
assigned to 251,252,253102, although the team wrote that formation of
isotopes from lead or bismuth impurities (which would not produce
nobelium) could not be ruled out. While later 1958 experiments noted
that new isotopes could be produced from mercury, thallium, lead, or
bismuth impurities, the scientists still stood by their conclusion
that element 102 could be produced from this reaction, mentioning a
half-life of under 30 seconds and a decay energy of (8.8 ± 0.5) MeV.
Later 1960 experiments proved that these were background effects. 1967
experiments also lowered the decay energy to (8.6 ± 0.4) MeV, but both
values are too high to possibly match those of 253No or 254No. The
Dubna team later stated in 1970 and again in 1987 that these results
were not conclusive.

In 1961, Berkeley scientists claimed the discovery of element 103 in
the reaction of californium with boron and carbon ions. They claimed
the production of the isotope 257103, and also claimed to have
synthesized an alpha decaying isotope of element 102 that had a
half-life of 15 s and alpha decay energy 8.2 MeV. They assigned this
to 255102 without giving a reason for the assignment. The values do
not agree with those now known for 255No, although they do agree with
those now known for 257No, and while this isotope probably played a
part in this experiment, its discovery was inconclusive.

Work on element 102 also continued in Dubna, and in 1964, experiments
were carried out there to detect alpha-decay daughters of element 102
isotopes by synthesizing element 102 from the reaction of a
uranium-238 target with neon ions. The products were carried along a
silver catcher foil and purified chemically, and the isotopes 250Fm
and 252Fm were detected. The yield of 252Fm was interpreted as
evidence that its parent 256102 was also synthesized: as it was noted
that 252Fm could also be produced directly in this reaction by the
simultaneous emission of an alpha particle with the excess neutrons,
steps were taken to ensure that 252Fm could not go directly to the
catcher foil. The half-life detected for 256102 was 8 s, which is much
higher than the more modern 1967 value of (3.2 ± 0.2) s. Further
experiments were conducted in 1966 for 254102, using the reactions
243Am(15N,4n)254102 and 238U(22Ne,6n)254102, finding a half-life of
(50 ± 10) s: at that time the discrepancy between this value and the
earlier Berkeley value was not understood, although later work proved
that the formation of the isomer 250mFm was less likely in the Dubna
experiments than at the Berkeley ones. In hindsight, the Dubna results
on 254102 were probably correct and can be now considered a conclusive
detection of element 102.

One more very convincing experiment from Dubna was published in 1966
(though it was submitted in 1965), again using the same two reactions,
which concluded that 254102 indeed had a half-life much longer than
the 3 seconds claimed by Berkeley. Later work in 1967 at Berkeley and
1971 at the Oak Ridge National Laboratory fully confirmed the
discovery of element 102 and clarified earlier observations. In
December 1966, the Berkeley group repeated the Dubna experiments and
fully confirmed them, and used this data to finally assign correctly
the isotopes they had previously synthesized but could not yet
identify at the time. Thus they claimed to have discovered nobelium in
1958 to 1961.

: +  →  →  + 6

In 1969, the Dubna team carried out chemical experiments on element
102 and concluded that it behaved as the heavier homologue of
ytterbium. The Russian scientists proposed the name 'joliotium' (Jo)
for the new element after Irène Joliot-Curie, who had recently died,
creating an element naming controversy that would not be resolved for
several decades, with each group using its own proposed names.

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
recognised as the discoverers of nobelium, although it is possible
that it was detected at Berkeley in 1959. This decision was criticized
by Berkeley the following year, calling the reopening of the cases of
elements 101 to 103 a "futile waste of time", while Dubna agreed with
IUPAC's decision.

In 1994, as part of an attempted resolution to the element naming
controversy, IUPAC ratified names for elements 101-109. For element
102, it ratified the name 'nobelium' (No) on the basis that it had
become entrenched in the literature over the course of 30 years and
that Alfred Nobel should be commemorated in this fashion. 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. Today the name 'flerovium', with the same
symbol, refers to element 114.


Physical
==========
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 lanthanides and actinides, in the metallic state, can exist as
either divalent (such as europium and ytterbium) or trivalent (most
other lanthanides) metals. The former have f'n's2 configurations,
whereas the latter have f'n'−1d1s2 configurations. In 1975, Johansson
and Rosengren examined the measured and predicted values for the
cohesive energies (enthalpies of crystallization) of the metallic
lanthanides and actinides, both as divalent and trivalent metals. 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. The increasing predominance of the divalent state well
before the actinide series concludes is attributed to the relativistic
stabilization of the 5f electrons, which increases with increasing
atomic number: an effect of this is that nobelium is predominantly
divalent instead of trivalent, unlike all the other lanthanides and
actinides. 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. Like the other divalent late
actinides (except the once again trivalent lawrencium), metallic
nobelium should assume a face-centered cubic crystal structure.
Divalent nobelium metal should have a metallic radius of around 197
pm. Nobelium's melting point has been predicted to be 800 °C, the same
value as that estimated for the neighboring element mendelevium. Its
density is predicted to be around 9.9 ± 0.4 g/cm3.


Chemical
==========
The chemistry of nobelium is incompletely characterized and is known
only in aqueous solution, in which it can take on the +3 or +2
oxidation states, the latter being more stable. 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. Determination of nobelium's favoring
of the +2 state had to wait until the next year, when cation-exchange
chromatography and coprecipitation experiments were carried out on
around fifty thousand 255No atoms, finding that it behaved differently
from the other actinides and more like the divalent alkaline earth
metals. This proved that in aqueous solution, nobelium is most stable
in the divalent state when strong oxidizers are absent. Later
experimentation in 1974 showed that nobelium eluted with the alkaline
earth metals, between Ca2+ and Sr2+. Nobelium is the only known
f-block element for which the +2 state is the most common and stable
one in aqueous solution. This occurs because of the large energy gap
between the 5f and 6d orbitals at the end of the actinide series.

It is expected that the relativistic stabilization of the 7s subshell
greatly destabilizes nobelium dihydride, NoH2, and relativistic
stabilisation of the 7p1/2 spinor over the 6d3/2 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 'n's2 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. Its complexing ability
with citrate, oxalate, and acetate in an aqueous solution of 0.5 M
ammonium nitrate is between that of calcium and strontium, although it
is somewhat closer to that of strontium.

The standard reduction potential of the 'E'°(No3+→No2+) couple was
estimated in 1967 to be between +1.4 and +1.5 V; it was later found in
2009 to be only about +0.75 V. 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. It has
been predicted that the value for the 'E'°(No4+→No3+) couple would be
+6.5 V. The Gibbs energies of formation for No3+ and No2+ are
estimated to be −342 and −480 kJ/mol, respectively.


Atomic
========
A nobelium atom has 102 electrons. They are expected to be arranged in
the configuration [Rn]5f147s2 (ground state term symbol 1S0), although
experimental verification of this electron configuration had not yet
been made as of 2006. The sixteen electrons in the 5f and 7s subshells
are valence electrons. In forming compounds, three valence electrons
may be lost, leaving behind a [Rn]5f13 core: this conforms to the
trend set by the other actinides with their [Rn]5f'n' electron
configurations in the tripositive state. Nevertheless, it is more
likely that only two valence electrons are lost, leaving behind a
stable [Rn]5f14 core with a filled 5f14 shell. The first ionization
potential of nobelium was measured to be at most (6.65 ± 0.07) eV in
1974, based on the assumption that the 7s electrons would ionize
before the 5f ones; this value has not yet been refined further due to
nobelium's scarcity and high radioactivity. The ionic radius of
hexacoordinate and octacoordinate No3+ had been preliminarily
estimated in 1978 to be around 90 and 102 pm respectively; the ionic
radius of No2+ has been experimentally found to be 100 pm to two
significant figures. The enthalpy of hydration of No2+ has been
calculated as 1486 kJ/mol.


Isotopes
==========
Fourteen isotopes of nobelium are known, with mass numbers 248-260 and
262; all are radioactive. Additionally, nuclear isomers are known for
mass numbers 250, 251, 253, and 254. 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 1.7 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
3.1 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. After 259No and 255No, the next
most stable nobelium isotopes are 253No (half-life 1.62 minutes),
254No (51 seconds), 257No (25 seconds), 256No (2.91 seconds), and
252No (2.57 seconds). All of the remaining nobelium isotopes have
half-lives that are less than a second, and the shortest-lived known
nobelium isotope (248No) has a half-life of less than 2 microseconds.
The isotope 254No is especially interesting theoretically as it is in
the middle of a series of prolate nuclei from 231Pa to 279Rg, and the
formation of its nuclear isomers (of which two are known) is
controlled by proton orbitals such as 2f5/2 which come just above the
spherical proton shell; it can be synthesized in the reaction of 208Pb
with 48Ca.

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.


                    Preparation and purification
======================================================================
The isotopes of nobelium are mostly produced by bombarding actinide
targets (uranium, plutonium, curium, californium, or einsteinium),
with the exception of nobelium-262, which is produced as the daughter
of lawrencium-262. The most commonly used isotope, 255No, can be
produced from bombarding curium-248 or californium-249 with carbon-12:
the latter method is more common. Irradiating a 350 μg cm−2 target of
californium-249 with three trillion (3 × 1012) 73 MeV carbon-12 ions
per second for ten minutes can produce around 1200 nobelium-255 atoms.

Once the nobelium-255 is produced, it can be separated out similarly
as used to purify the neighboring actinide mendelevium. The recoil
momentum of the produced nobelium-255 atoms is used to bring them
physically far away from the target from which they are produced,
bringing them onto a thin foil of metal (usually beryllium, aluminium,
platinum, or gold) just behind the target in a vacuum: this is usually
combined by trapping the nobelium atoms in a gas atmosphere
(frequently helium), and carrying them along with a gas jet from a
small opening in the reaction chamber. Using a long capillary tube,
and including potassium chloride aerosols in the helium gas, the
nobelium atoms can be transported over tens of meters. The thin layer
of nobelium collected on the foil can then be removed with dilute acid
without completely dissolving the foil. The nobelium can then be
isolated by exploiting its tendency to form the divalent state, unlike
the other trivalent actinides: under typically used elution conditions
(bis-(2-ethylhexyl) phosphoric acid (HDEHP) as stationary organic
phase and 0.05 M hydrochloric acid as mobile aqueous phase, or using 3
M hydrochloric acid as an eluant from cation-exchange resin columns),
nobelium will pass through the column and elute while the other
trivalent actinides remain on the column. However, if a direct
"catcher" gold foil is used, the process is complicated by the need to
separate out the gold using anion-exchange chromatography before
isolating the nobelium by elution from chromatographic extraction
columns using HDEHP.


                           External links
======================================================================
* [http://www.nndc.bnl.gov/chart/ Chart of Nuclides] . nndc.bnl.gov
* [http://periodic.lanl.gov/102.shtml Los Alamos National Laboratory -
Nobelium]
* [http://www.periodicvideos.com/videos/102.htm Nobelium] at 'The
Periodic Table of Videos' (University of Nottingham)


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