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= Rutherfordium =
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Introduction
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Rutherfordium is a synthetic chemical element; it has symbol Rf and
atomic number 104. It is named after physicist Ernest Rutherford. As a
synthetic element, it is not found in nature and can only be made in a
particle accelerator. It is radioactive; the most stable known
isotope, 267Rf, has a half-life of about 48 minutes.
In the periodic table, it is a d-block element and the second of the
fourth-row transition elements. It is in period 7 and is a group 4
element. Chemistry experiments have confirmed that rutherfordium
behaves as the heavier homolog to hafnium in group 4. The chemical
properties of rutherfordium are characterized only partly. They
compare well with the other group 4 elements, even though some
calculations had indicated that the element might show significantly
different properties due to relativistic effects.
In the 1960s, small amounts of rutherfordium were produced at Joint
Institute for Nuclear Research in the Soviet Union and at Lawrence
Berkeley National Laboratory in California. Priority of discovery and
hence the name of the element was disputed between Soviet and American
scientists, and it was not until 1997 that the International Union of
Pure and Applied Chemistry (IUPAC) established rutherfordium as the
official name of the element.
Discovery
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Rutherfordium was reportedly first detected in 1964 at the Joint
Institute for Nuclear Research at Dubna (Soviet Union at the time).
Researchers there bombarded a plutonium-242 target with neon-22 ions;
a spontaneous fission activity with half-life 0.3 ± 0.1 seconds was
detected and assigned to 260104. Later work found no isotope of
element 104 with this half-life, so that this assignment must be
considered incorrect.
In 1966-1969, the experiment was repeated. This time, the reaction
products by gradient thermochromatography after conversion to
chlorides by interaction with ZrCl4. The team identified spontaneous
fission activity contained within a volatile chloride portraying
eka-hafnium properties.
: + → 264−'x'104 → 264−'x'104Cl4
The researchers considered the results to support the 0.3 second
half-life. Although it is now known that there is no isotope of
element 104 with such a half-life, the chemistry does fit that of
element 104, as chloride volatility is much greater in group 4 than in
group 3 (or the actinides).
In 1969, researchers at University of California, Berkeley
conclusively synthesized the element by bombarding a californium-249
target with carbon-12 ions and measured the alpha decay of 257104,
correlated with the daughter decay of nobelium-253:
: + → 257104 + 4
They were unable to confirm the 0.3-second half-life for 260104, and
instead found a 10-30 millisecond half-life for this isotope, agreeing
with the modern value of 21 milliseconds. In 1970, the American team
chemically identified element 104 using the ion-exchange separation
method, proving it to be a group 4 element and the heavier homologue
of hafnium.
The American synthesis was independently confirmed in 1973 and secured
the identification of rutherfordium as the parent by the observation
of K-alpha X-rays in the elemental signature of the 257104 decay
product, nobelium-253.
Naming controversy
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As a consequence of the initial competing claims of discovery, an
element naming controversy arose. Since the Soviets claimed to have
first detected the new element they suggested the name 'kurchatovium'
(Ku) in honor of Igor Kurchatov (1903-1960), former head of Soviet
nuclear research. This name had been used in books of the Soviet Bloc
as the official name of the element. The Americans, however, proposed
'rutherfordium' (Rf) for the new element to honor New Zealand
physicist Ernest Rutherford, who is known as the "father" of nuclear
physics. In 1992, the IUPAC/IUPAP Transfermium Working Group (TWG)
assessed the claims of discovery and concluded that both teams
provided contemporaneous evidence to the synthesis of element 104 in
1969, and that credit should be shared between the two groups. In
particular, this involved the TWG performing a new retrospective
reanalysis of the Russian work in the face of the later-discovered
fact that there is no 0.3-second isotope of element 104: they
reinterpreted the Dubna results as having been caused by a spontaneous
fission branch of 259104.
The American group wrote a scathing response to the findings of the
TWG, stating that they had given too much emphasis on the results from
the Dubna group. In particular they pointed out that the Russian group
had altered the details of their claims several times over a period of
20 years, a fact that the Russian team does not deny. They also
stressed that the TWG had given too much credence to the chemistry
experiments performed by the Russians, considered the TWG's
retrospective treatment of the Russian work based on unpublished
documents to have been "highly irregular", noted that there was no
proof that 259104 had a spontaneous fission branch at all (as of 2021
there still is not), and accused the TWG of not having appropriately
qualified personnel on the committee. The TWG responded by saying that
this was not the case and having assessed each point raised by the
American group said that they found no reason to alter their
conclusion regarding priority of discovery.
The International Union of Pure and Applied Chemistry (IUPAC) adopted
'unnilquadium' (Unq) as a temporary, systematic element name, derived
from the Latin names for digits 1, 0, and 4. In 1994, IUPAC suggested
a set of names for elements 104 through 109, in which 'dubnium' (Db)
became element 104 and 'rutherfordium' became element 106. This
recommendation was criticized by the American scientists for several
reasons. Firstly, their suggestions were scrambled: the names
'rutherfordium' and 'hahnium', originally suggested by Berkeley for
elements 104 and 105, were respectively reassigned to elements 106 and
108. Secondly, elements 104 and 105 were given names favored by JINR,
despite earlier recognition of LBL as an equal co-discoverer for both
of them. Thirdly and most importantly, IUPAC rejected the name
'seaborgium' for element 106, having just approved a rule that an
element could not be named after a living person, even though the
IUPAC had given the LBNL team the sole credit for its discovery. In
1997, IUPAC renamed elements 104 to 109, and gave elements 104 and 106
the Berkeley proposals 'rutherfordium' and 'seaborgium'. The name
'dubnium' was given to element 105 at the same time. The 1997 names
were accepted by researchers and became the standard.
Isotopes
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Rutherfordium has no stable or naturally occurring isotopes. Several
radioactive isotopes have been synthesized in the laboratory, either
by fusing two atoms or by observing the decay of heavier elements.
Seventeen different isotopes have been reported with atomic masses
from 252 to 270 (with the exceptions of 264 and 269). Most of these
decay predominantly through spontaneous fission, particularly isotopes
with even neutron numbers, while some of the lighter isotopes with odd
neutron numbers also have significant alpha decay branches.
Stability and half-lives
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Out of isotopes whose half-lives are known, the lighter isotopes
usually have shorter half-lives. The three lightest known isotopes
have half-lives of under 50 μs, with the lightest reported isotope
252Rf having a half-life shorter than one microsecond. The isotopes
256Rf, 258Rf, 260Rf are more stable at around 10 ms; 255Rf, 257Rf,
259Rf, and 262Rf live between 1 and 5 seconds; and 261Rf, 265Rf, and
263Rf are more stable, at around 1.1, 1.5, and 10 minutes
respectively. The most stable known isotope, 267Rf, is one of the
heaviest, and has a half-life of about 48 minutes. Rutherfordium
isotopes with an odd neutron number tend to have longer half-lives
than their even-even neighbors because the odd neutron provides
additional hindrance against spontaneous fission.
The lightest isotopes were synthesized by direct fusion between two
lighter nuclei and as decay products. The heaviest isotope produced by
direct fusion is 262Rf; heavier isotopes have only been observed as
decay products of elements with larger atomic numbers. The heavy
isotopes 266Rf and 268Rf have also been reported as electron capture
daughters of the dubnium isotopes 266Db and 268Db, but have short
half-lives to spontaneous fission. It seems likely that the same is
true for 270Rf, a possible daughter of 270Db. These three isotopes
remain unconfirmed.
In 1999, American scientists at the University of California,
Berkeley, announced that they had succeeded in synthesizing three
atoms of 293Og. These parent nuclei were reported to have successively
emitted seven alpha particles to form 265Rf nuclei, but their claim
was retracted in 2001. This isotope was later discovered in 2010 as
the final product in the decay chain of 285Fl.
Predicted properties
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Very few properties of rutherfordium or its compounds have been
measured; this is due to its extremely limited and expensive
production and the fact that rutherfordium (and its parents) decays
very quickly. A few singular chemistry-related properties have been
measured, but properties of rutherfordium metal remain unknown and
only predictions are available.
Chemical
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Rutherfordium is the first transactinide element and the second member
of the 6d series of transition metals. Calculations on its ionization
potentials, atomic radius, as well as radii, orbital energies, and
ground levels of its ionized states are similar to that of hafnium and
very different from that of lead. Therefore, it was concluded that
rutherfordium's basic properties will resemble those of other group 4
elements, below titanium, zirconium, and hafnium. Some of its
properties were determined by gas-phase experiments and aqueous
chemistry. The oxidation state +4 is the only stable state for the
latter two elements and therefore rutherfordium should also exhibit a
stable +4 state. In addition, rutherfordium is also expected to be
able to form a less stable +3 state. The standard reduction potential
of the Rf4+/Rf couple is predicted to be higher than −1.7 V.
Initial predictions of the chemical properties of rutherfordium were
based on calculations which indicated that the relativistic effects on
the electron shell might be strong enough that the 7p orbitals would
have a lower energy level than the 6d orbitals, giving it a valence
electron configuration of 6d1 7s2 7p1 or even 7s2 7p2, therefore
making the element behave more like lead than hafnium. With better
calculation methods and experimental studies of the chemical
properties of rutherfordium compounds it could be shown that this does
not happen and that rutherfordium instead behaves like the rest of the
group 4 elements. Later it was shown in ab initio calculations with
the high level of accuracy that the Rf atom has the ground state with
the 6d2 7s2 valence configuration and the low-lying excited 6d1 7s2
7p1 state with the excitation energy of only 0.3-0.5 eV.
In an analogous manner to zirconium and hafnium, rutherfordium is
projected to form a very stable, refractory oxide, RfO2. It reacts
with halogens to form tetrahalides, RfX4, which hydrolyze on contact
with water to form oxyhalides RfOX2. The tetrahalides are volatile
solids existing as monomeric tetrahedral molecules in the vapor phase.
In the aqueous phase, the Rf4+ ion hydrolyzes less than titanium(IV)
and to a similar extent as zirconium and hafnium, thus resulting in
the RfO2+ ion. Treatment of the halides with halide ions promotes the
formation of complex ions. The use of chloride and bromide ions
produces the hexahalide complexes and . For the fluoride complexes,
zirconium and hafnium tend to form hepta- and octa- complexes. Thus,
for the larger rutherfordium ion, the complexes , and are possible.
Physical and atomic
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Rutherfordium is expected to be a solid under normal conditions and
have a hexagonal close-packed crystal structure ('c'/'a' = 1.61),
similar to its lighter congener hafnium. It should be a metal with
density ~17 g/cm3. The atomic radius of rutherfordium is expected to
be ~150 pm. Due to relativistic stabilization of the 7s orbital and
destabilization of the 6d orbital, Rf+ and Rf2+ ions are predicted to
give up 6d electrons instead of 7s electrons, which is the opposite of
the behavior of its lighter homologs. When under high pressure
(variously calculated as 72 or ~50 GPa), rutherfordium is expected to
transition to body-centered cubic crystal structure; hafnium
transforms to this structure at 71±1 GPa, but has an intermediate ω
structure that it transforms to at 38±8 GPa that should be lacking for
rutherfordium.
Gas phase
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Early work on the study of the chemistry of rutherfordium focused on
gas thermochromatography and measurement of relative deposition
temperature adsorption curves. The initial work was carried out at
Dubna in an attempt to reaffirm their discovery of the element. Recent
work is more reliable regarding the identification of the parent
rutherfordium radioisotopes. The isotope 261mRf has been used for
these studies, though the long-lived isotope 267Rf (produced in the
decay chain of 291Lv, 287Fl, and 283Cn) may be advantageous for future
experiments. The experiments relied on the expectation that
rutherfordium would be a 6d element in group 4 and should therefore
form a volatile molecular tetrachloride, that would be tetrahedral in
shape. Rutherfordium(IV) chloride is more volatile than its lighter
homologue hafnium(IV) chloride (HfCl4) because its bonds are more
covalent.
A series of experiments confirmed that rutherfordium behaves as a
typical member of group 4, forming a tetravalent chloride (RfCl4) and
bromide (RfBr4) as well as an oxychloride (RfOCl2). A decreased
volatility was observed for when potassium chloride is provided as
the solid phase instead of gas, highly indicative of the formation of
nonvolatile mixed salt.
Aqueous phase
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Rutherfordium is expected to have the electron configuration [Rn]5f14
6d2 7s2 and therefore behave as the heavier homologue of hafnium in
group 4 of the periodic table. It should therefore readily form a
hydrated Rf4+ ion in strong acid solution and should readily form
complexes in hydrochloric acid, hydrobromic or hydrofluoric acid
solutions.
The most conclusive aqueous chemistry studies of rutherfordium have
been performed by the Japanese team at Japan Atomic Energy Research
Institute using the isotope 261mRf. Extraction experiments from
hydrochloric acid solutions using isotopes of rutherfordium, hafnium,
zirconium, as well as the pseudo-group 4 element thorium have proved a
non-actinide behavior for rutherfordium. A comparison with its lighter
homologues placed rutherfordium firmly in group 4 and indicated the
formation of a hexachlororutherfordate complex in chloride solutions,
in a manner similar to hafnium and zirconium.
: + 6 →
Very similar results were observed in hydrofluoric acid solutions.
Differences in the extraction curves were interpreted as a weaker
affinity for fluoride ion and the formation of the
hexafluororutherfordate ion, whereas hafnium and zirconium ions
complex seven or eight fluoride ions at the concentrations used:
: + 6 →
Experiments performed in mixed sulfuric and nitric acid solutions
shows that rutherfordium has a much weaker affinity towards forming
sulfate complexes than hafnium. This result is in agreement with
predictions, which expect rutherfordium complexes to be less stable
than those of zirconium and hafnium because of a smaller ionic
contribution to the bonding. This arises because rutherfordium has a
larger ionic radius (76 pm) than zirconium (71 pm) and hafnium (72
pm), and also because of relativistic stabilisation of the 7s orbital
and destabilisation and spin-orbit splitting of the 6d orbitals.
Coprecipitation experiments performed in 2021 studied rutherfordium's
behaviour in basic solution containing ammonia or sodium hydroxide,
using zirconium, hafnium, and thorium as comparisons. It was found
that rutherfordium does not strongly coordinate with ammonia and
instead coprecipitates out as a hydroxide, which is probably Rf(OH)4.
External links
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*
* [
http://www.periodicvideos.com/videos/104.htm Rutherfordium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.webelements.com/webelements/elements/text/Rf/index.html
WebElements.com - Rutherfordium]
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Original Article:
http://en.wikipedia.org/wiki/Rutherfordium
