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Rutherfordium (pronounced /ˌrʌðərˈfɔrdiəm/ RUDH-ər-FOR-dee-əm) is a chemical element with the symbol Rf and atomic number 104. On the periodic table of the elements, it is a p-block element and the first one of the transactinide element. It is member of the 7th period and also belongs to the group 4 elements. Chemistry experiments have confirmed that rutherfordium behaves as the heavier homologue to hafnium in group 4. Rutherfordium is a radioactive synthetic element whose most stable known isotope is 267Rf with a half-life of approximately 1.3 hours.
Small amounts of rutherfordium have been produced by bombarding plutonium-242 with accelerated neon-22 or californium-249 with accelerated carbon-12 ions in the 1960s. The priority of the discovery and therefore the naming of the element was disputed between Russian and American scientists and a final decision was taken in 1997 naming the element rutherfordium to honor New Zealand physicist Ernest Rutherford. Improved experimental techniques allowed to characterize some chemical properties of rutherfordium, which fit well into the chemistry of the other group 4 elements. Some calculations indicated that the element might show significantly different properties due to relativistic effects.
In 1969 researchers at the 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 253102.
The Russian scientists proposed the name Kurchatovium for the new element. The American scientists proposed the name Rutherfordium for the new element.
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 and that credit should be shared between the two groups.
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 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 IUPAC finally used the name suggested by the American team (rutherfordium) which may in some way reflect a change of opinion.
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 Ernest Rutherford, who is known as the "father" of nuclear physics. 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 the name dubnium to be used since rutherfordium was suggested for element 106 and IUPAC felt that the Dubna team should be rightly recognized for their contributions. However, there was still a dispute over the names of elements 104−107. However in 1997 the teams involved resolved the dispute and adopted the current name rutherfordium.
The chemical properties of rutherfordium were based on calculation which indicated that the relativistic effects on the electron shell might be strong enough that the p orbitals have a lower energy level then the s orbitals and therefore the element more behaves like lead. With better calculation methods and studies of the chemical properties of rutherfordium compounds it could be shown that rutherfordium behaves according to the rest of the group 4 elements.
This section deals with the synthesis of nuclei of rutherfordium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.
After an upgrade of their facilities, they repeated the reaction in 1994 with much higher sensitivity and detected some 1100 atoms of 257Rf and 1900 atoms of 256Rf along with 255Rf in the measurement of the 1n,2n and 3n excitation functions. The large amount of decay data for 257Rf allowed the detection of an isomeric level and the construction of a partial decay level structure which confirmed the very complicated alpha decay pattern. They also found evidence for an isomeric level in 255Rf.  The GSI team continued in 2001 with the measurement of the 3n excitation function. In 2002, scientists at the Argonne University in Illinois began their first studies of translawrencium elements with the synthesis and alpha-gamma spectroscopy of 257Rf. In 2004, the GSI began their spectroscopic studies of the 257Rf isotope. In 2007, the Lawrence Berkeley National Laboratory (LBNL) studied the 2n product, 256Rf, in a search for K-isomers and discovered three such isomers.
The team at GSI first studied this reaction in 1994 in an effort to study neutron deficient isotopes of rutherfordium. They were able to detect 255Rf and 144 atoms of the new isotope 254Rf, which decayed by spontaneous fission.
The team at GSI first studied this reaction in 1994 in an effort to study neutron deficient isotopes of rutherfordium. They were able to detect 14 atoms of the new isotope 253Rf, which decayed by spontaneous fission.
In an important study, in May 2004, the team at GSI attempted the symmetric synthesis of rutherfordium by attempting to fuse two fission fragments. Theory suggests that there may be an enhancement of the yield. No product atoms were detected and an upper limit of 1000 pb was estimated for the yield of this reaction.
This section deals with the synthesis of nuclei of rutherfordium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons.
The hot fusion reaction using a uranium target was first reported in 2000 by Yuri Lazarev and the team at the Flerov Laboratory of Nuclear Reactions (FLNR). They were able to observe decays from 260Rf and 259Rf in the 4n and 5n channels.  They measured yields of 240 pb in the 4n channel and 1.1 nb in the 5n channel. In 2006, as part of their program on the study of uranium targets in hot fusion reactions, the team at LBNL measured the 4n,5n and 6n excitation functions for this reaction and observed 261Rf in the 3n exit channel.
This reaction was reported in 1996 at LBNL in an attempt to study the fission characteristics of 262Rf. The team were able to detect the spontaneous fission (SF) of 262Rf and determine its half-life as 2.1 s, in contrast to earlier reports of a 47 ms activity. It was suggested that the two half-lives might be related to different isomeric states. The reaction was further studied in 2000 by Yuri Lazarev and the team at Dubna. They were able to observe 69 alpha decays from 261Rf and spontaneous fission of 262Rf. Later work on hassium has allowed a reassignment of the 5n product to 261mRf.
The synthesis of element 104 was first attempted in 1964 by the team at Dubna using this reaction. The first study produced evidence for a 0.3 s SF activity tentatively assigned to 260104 or 259104 and an unidentified 8 s SF activity. The former activity was later retracted and the latter activity associated with the now-known 259104. In 1966, in their discovery experiment, the team repeated the reaction using a chemical study of volatile chloride products. The group was able to identify a volatile chloride decaying by short spontaneous fission with eka-hafnium properties. This gave strong evidence for the formation of Cl4 and the team suggested the name kurchatovium. Although a half-life was not accurately measured, later evidence suggested that the product was most likely 259104.  In 1968, the team searched for alpha decay from 260104 but were unable to detect such activity. In 1970, the team repeated the reaction once again and confirmed the ~0.2 s SF activity. They also repeated the chemistry experiment and obtained identical results to their 1966 experiment and calculated a likely half-life of ~0.5 seconds for the SF activity. In 1971, the reaction was repeated again and 0.1 s and 4.5 s SF activities were found. The 4.5 s activity was correctly assigned to 259104. A chemistry experiment in the same year reaffirmed the formation of a 0.3 SF activity for an eka-hafnium product. Later, the 0.1−0.3 s SF activity was retracted as belonging to a kurchatovium isotope but the observation of eka-hafnium reactivity remained and was the basis of their successful claim to discovery. The reaction was further studied in 2000 by Yuri Lazarev at Dubna. They were able to observe 261Rf in the 3n channel, later reassigned to 261mRf.
This reaction was first studied in 1964 to assist in the assignments using the analogous reaction with a Ne-22 beam. The Dubna team were unable to detect any 0.3 s spontaneous fission activities. The reaction was later studied in 2003 at the Paul Scherrer Institute (PSI) in Bern, Switzerland. They detected some spontaneous fission activities but were unable to confirm the formation of 259Rf.
This reaction was studied in 1999 at the University of Bern, Switzerland in order to search for the new isotope 263Rf. A rutherfordium fraction was separated and several SF events with long lifetimes and alpha decays with energy 7.8 MeV and 7.9 MeV were observed. A second experiment using a study of the fluoride of rutherfordium products also produced 7.9 MeV alpha decays.
This reaction was first studied in 1970 by Albert Ghiorso at the LBNL. The team identified 261Rf in the 5n channel using the method of correlation of genetic parent-daughter decays. A half-life of 65 s was determined.  A repeat later that year using cation exchange chromatography indicated that the product did not form a +2 or +3 cation and behaved as eka-hafnium. A study of the properties of rutherfordium isotopes was performed in 1981 at the LBNL. In a series of reactions, a 1.5 s SF activity was identified and assigned to a fermium descendant although later evidence indicates a possible assignment to 262Rf. In contrast, in a subsequent review of isotope properties by Somerville et al. at LBNL in 1985, a 47 ms SF activity was assigned to 262Rf. This assignment has not been verified.  The reaction was further studied in 1991 by Czerwinski et al. at the LBNL. In this experiment, spontaneous fission activities with long lifetimes were observed in rutherfordium fractions and tentatively assigned to 263Rf. In 1996, chemical studies on the chloride of rutherfordium were published by the LBNL. In this experiment, the half-life was improved to 78 s. A repeat of the experiment in 2000 assessing the volatility of the bromide further refined the half-life to 75 s.
This reaction was studied in 1969 by Albert Ghiorso at the University of California. The aim was to detect the 0.1–0.3 s SF activity reported at Dubna, assigned to 260104. They were unable to do so, only observing a 10–30 ms SF activity, correctly assigned to 260104. The failure to observe the 0.3 s SF activity identified by Dubna gave the Americans the incentive to name this element rutherfordium.
In an attempt to unravel the properties of spontaneous fission activities in the formation of rutherfordium isotopes, this reaction was performed in 1976 by the FLNR. They observed an 80 ms SF activity. Subsequent work led to the complete retraction of the 0.3s - 0.1s - 80 ms SF activities observed by the Dubna team and associated with background signals.
This reaction was studied in 1977 by the team in Dubna. They were able to confirm the detection of a 76 ms SF activity. The assignment to rutherfordium isotopes was later retracted. The LBNL re-studied the reaction in 1980 and in 1981 they reported that they were unable to confirm the ~80 ms SF activity. The Dubna team were able to measure a 28 ms SF activity in 1985 and assigned the isotope correctly to 260104.
This use of californium-249 as a target was first studied by Albert Ghiorso and the team at the University of California in 1969. They were able to observe an 11 ms SF activity which they correctly assigned to 258104.
In their 1969 discovery experiments, the team at University of California also used carbon-12 beam to irradiate a californium-249 target. They were able to confirm the 11 ms SF activity found with a carbon-13 beam and again correctly assigned to 258104. The actual discovery experiment was the observation of alpha decays genetically linked to 253102 and therefore positively identified as 257104. In 1973, Bemis and his team at Oak Ridge confirmed the discovery by measuring coincident X-rays from the daughter 253102.
Isotopes of rutherfordium have also been identified in the decay of heavier elements. Observations to date are summarized in the table below. EC refers to electron capture.
In 1999, American scientists at the University of California, Berkeley, announced that they had succeeded in synthesizing three atoms of 293118. These parent nuclei successively emitted seven alpha particles to form 265Rf nuclei. Their claim was retracted in 2001. As such, this rutherfordium isotope is unconfirmed or unknown.
A detailed spectroscopic study of the production of 255Rf nuclei using the reaction 206Pb(50Ti,n)255Rf allowed the tentative identification of an isomeric level in 255Rf. A more detailed study later confirmed that this was not the case.
Initial work on the synthesis of rutherfordium isotopes by hot fusion pathways focused on the synthesis of 263Rf. Several studies have indicated that this nuclide decays primarily by spontaneous fission with a long half-life of 10–20 minutes. Alpha particles with energy 7.8−7.9 MeV have also been associated with this nucleus. More recently, a study of hassium isotopes allowed the synthesis of an atom of 263Rf decaying by spontaneous fission with a short half-life of 8 seconds. These two different decay modes must be associated with two isomeric states. Specific assignments are difficult due to the low number of observed events. Further studies are required to allow definite assignments.
Early research on the synthesis of rutherfordium isotopes utilized the 244Pu(22Ne,5n)261Rf reaction. The product was found to undergo exclusive 8.28 MeV alpha decay with a half-life of 78 seconds. Later studies by the GSI team on the synthesis of copernicium and hassium isotopes produced conflicting data. In this case, 261Rf was found to undergo 8.52 MeV alpha decay with a short half-life of 4 seconds. Later results indicated a predominant fission branch. These contradictions led to some doubt on the discovery of copernicium. The first isomer is currently denoted 261aRf whilst the second is denoted 261bRf. However, it is thought that the first nucleus belongs to a high-spin ground state and the latter to a low-spin metastable state.  The discovery and confirmation of 261bRf provided proof for the discovery of copernicium in 1996.
A detailed spectroscopic study of the production of 257Rf nuclei using the reaction 208Pb(50Ti,n)257Rf allowed the identification of an isomeric level in 257Rf. The work confirmed that 257gRf has a very complicated spectrum with as many as 15 alpha lines. A level structure diagram was calculated for both isomers.
The table below provides cross-sections and excitation energies for cold fusion reactions producing rutherfordium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel. Yields are expressed in nanobarns (nb) and picobarns (pb).
The table below provides cross-sections and excitation energies for hot fusion reactions producing rutherfordium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.
Element 104 is the first member of the 6d series of transition metals and the heaviest member of group IV in the periodic table, below titanium, zirconium and hafnium. It may also be named eka-hafnium or dvi-zirconium and some of its properties were determined by gas-phase experiments and aqueous chemistry. The IV oxidation state is the only stable state for the latter two elements and therefore rutherfordium should also portray a stable +4 state.
In an analogous manner to zirconium and hafnium, rutherfordium is projected to form a very stable, high melting point 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 hydrolyze less than titanium(IV) and to a similar extent to zirconium and hafnium, thus leading to the rutherfordyl oxyion, RfO2+. Treatment of the halides with halide ions promotes the formation of complex ions. The use of chloride and bromide ion form the hexahalide complexes RfCl62− and RfBr62−. For the fluoride complexes, zirconium and hafnium tend to form hepta- and octa- complexes. Thus, for the larger rutherfordium ion, the complexes RfF62−, RfF73− and RfF84− are possible.
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. The experiments relied on the expectation that rutherfordium would begin the new 6d series of elements and should therefore form a volatile tetrachloride due to the tetrahedral nature of the molecule.
As series of experiments have 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).
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.
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