Rutherfordium

lawrencium ← rutherfordium → dubnium Hf ↑ Rf ↓ (Upq) 104Rf Periodic table
Appearance
unknown
General properties
Name, symbol, number	rutherfordium, Rf, 104
Pronunciation	/ˌrʌðərˈfɔrdiəm/ RUDH-ər-FOR-dee-əm
Element category	transition metal
Group, period, block	4, 7, d
Standard atomic weight	[261.11]g·mol−1
Electron configuration	[Rn] 5f14 6d2 7s2 (predicted)[1]
Electrons per shell	2, 8, 18, 32, 32, 10, 2 (predicted) (Image)
Physical properties
Phase	solid (predicted)[1]
Density (near r.t.)	23 (estimated)[1] g·cm−3
Melting point	2400 (estimated)[1] K, 2100 °C
Boiling point	5800 (estimated)[1] K, 5500 °C
Atomic properties
Oxidation states	+4 (predicted)[1]
Ionization energies	1st: 490 (estimated)[1] kJ·mol−1
Miscellanea
CAS registry number	53850-36-5
Most stable isotopes
Main article: Isotopes of rutherfordium
iso NA half-life DM DE (MeV) DP 255Rf syn 1.68 s[2] 52% SF 48% α 8.81,8.77,8.74,8.71 251No <1% ε  ? 259Lw 257Rf syn 4.7 s[2] <100% α 8.90,8.78,8.52,8.28 253No <1.4% SF 257m1Rf syn 3.9 s[2] <100% α 9.02,8.97 253No <1.4% SF 259Rf syn 3.2 s[2] 92% α 8.87,8.77 255No 8% SF 261Rf syn 65 s[2] >80% α 8.28 257No <15% ε 261Lw <10% SF] 263Rf syn ~10 m[2] <100% SF ~30% α 7.90 ? 264Rf? syn 1 h?[2] α? 265Rf syn ~13 h[2] α? 266Rf? syn 10 h?[2] SF?/α? 268Rf? syn 6 h?[2] SF?/α?

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.

History

Discovery

Element 104 was reportedly first detected in 1966 at the Joint Institute of Nuclear Research at Dubna (then in Soviet Union). Researchers there bombarded ²⁴²Pu with accelerated ²²Ne ions and separated the reaction products by gradient thermochromatography after conversion to chlorides by interaction with ZrCl₄. The team identified a spontaneous fission activity contained within a volatile chloride portraying eka-hafnium properties. Although a half-life was not accurately determined, later calculations indicated that the product was most likely ²⁵⁹Rf:^[3]

24294Pu + 2210Ne → 264−x104Rf → 264−x104RfCl₄

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.^[4]

24998Cf + 126C → 257104Rf + 4 n

The American synthesis was independently confirmed in 1973 and secured the identification of element 104 as the parent by the observation of K-alpha X-rays in the elemental signature of the daughter ²⁵³No. ^[5]

Naming controversy

The Russian scientists proposed the name kurchatovium and the American scientists suggested the name rutherfordium for the new element.^[6] 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.^[3]

The 104^th element was eventually named after Ernest Rutherford (left side) (1871–1937), a British-New Zealand chemist and physicist who became known as the father of nuclear physics, even though the it was initially proposed to be named after Igor Kurchatov (right side) (1903–1960), a Soviet nuclear physicist who is remembered as "The Father of the Soviet Atomic Bomb".

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.^[7] The IUPAC finally used the name suggested by the American team (rutherfordium) which may in some way reflect a change of opinion.^[8]

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.^[8]

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.^[9]

Nucleosynthesis

Super heavy elements such as rutherfordium are produced by the bombardment of lighter elements using particle accelerators namely using fusion reactions to create them. Depending on the energies involved, these can be either cold fusion or hot fusion reactions. Whereas most of the isotopes of rutherfordium can be synthesized directly this way, some of the heavier ones have only been observed as decay products of elements with higher atomic numbers.

Hot fusion studies

In hot fusion reactions, very heavy targets (actinides) together with very light, high-energy projectiles are used. This gives rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. The synthesis of element 104 was first attempted in 1964 by the team at Dubna using the hot fusion reaction of neon projectiles with plutonium targets in the fusion reaction:

24294Pu + 2210Ne → 264-x104Rf + x n (x=3, 4?, or 5?)

The first study produced evidence for a spontaneous fission (SF) activity with a 0.3 second half-life, tentatively assigned to ²⁶⁰104 or ²⁵⁹104, and an unidentified SF activity at 8 seconds half-time. The former activity was later retracted and the latter activity associated with the now-known ²⁵⁹104 isotope.^[3]

In 1966, in their discovery experiment, the team repeated the same reaction using a chemical study of volatile chloride products. The group identified a volatile chloride decaying by short spontaneous fission with eka-hafnium properties. This gave strong evidence for the formation of [104]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.^[3] In 1968, the team searched for alpha decay from ²⁶⁰104 but were unable to detect such activity, but in 1970 repeated the chemistry experiment and obtained identical results to their 1966 experiment. 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.^[3] 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.^[3] The reaction was further studied in 2000 by Yuri Lazarev at Dubna who observed ²⁶¹Rf, later reassigned to ^261mRf. A similar reaction was studied in 1964 by the researchers at Dubna to assist in the original assignments, using a different neon-20 isotope, but were unable to detect any 0.3 s spontaneous fission activities.^[3] Later studies in 2003 at the Paul Scherrer Institute (PSI) in Bern, detected some SF activities but were unable to confirm the formation of ²⁵⁹Rf.^[10]

Researchers at the University of California led by Albert Ghiorso tried to detect in 1969 the 0.1–0.3 s SF activity reported at Dubna, assigned to ²⁶⁰104, from a curium and oxygen reaction:

24896Cm + 168O → 264-x104Rf + x n (x=4).

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. Later experiments in 1976 with curium-246, lead the Americans to associate the Soviet's original observations to background signals.^[3] In 1969, the Californian team also investigated the reaction of californium with carbon and observed an 11 ms SF activity which they correctly assigned to ²⁵⁸104:^[4]

24998Cf + 136C → 262-x104Rf + x n (x=4).

In their 1969 discovery experiments, the team at University of California also used carbon-12 beam instead of carbon-13 to irradiate a californium-249 target, and they confirmed the 11 ms SF activity assigned to ²⁵⁸104. However, the actual discovery experiment was the observation of alpha decays linked to ²⁵³102, and therefore positively identified as ²⁵⁷104.^[4] In 1973, Bemis and his team at Oak Ridge confirmed the discovery by measuring coincident X-rays from the daughter ²⁵³102.^[5]

In 1970 Albert Ghiorso's team also studied the reaction of curium with oxygen-18 and identified ²⁶¹Rf with a half life of 65 seconds.^[11] In a 1981 study at LBNL, a 1.5 s SF activity was identified and assigned to a fermium descendant, but 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, but this has not been verified.^[12] Further studies in 1991 at the LBNL indicated spontaneous fission activities with long lifetimes, tentatively assigned to ²⁶³Rf. In 1996, chemical studies on rutherfordium chloride were published by the LBNL, in which 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.

The reaction of berkelium-249 with nitrogen-15 was studied first in 1977 in Dubna, and eventually they confirmed in 1985 the formation of the ²⁶⁰104 isotope with a SF activity of 28 ms:^[3]

24997Bk + 147N → 264-x104Rf + x n (x=4).

Heavier isotopes were synthesized only much later. In 1996, the isotope ²⁶²Rf was first observed in LBNL from the reaction:

24494Pu + 2210Ne → 266-x104Rf + x n (x=4, 5).

The team detected 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.^[13] The reaction was further studied in 2000 by Yuri Lazarev and the team at Dubna which observed 69 alpha decays from ²⁶¹Rf and spontaneous fission of ²⁶²Rf.^[14] Later work on hassium has allowed a reassignment of the former product to ^261mRf.

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):

23892U + 2612Mg → 264-x104Rf + x n (x=3, 4, 5, 6).

They observed decays from ²⁶⁰Rf and ²⁵⁹Rf.^[15] In 2006, as part of their program on the study of uranium targets in hot fusion reactions, the team at LBNL observed also ²⁶¹Rf.^[16][17]

Cold fusion studies

In cold fusion reactions, the produced fused nuclei have a low excitation energy (~10-20 MeV) that increased the probability of these products to survive fission. As the fused nuclei cool to the ground state, they only require the emission of only one or two neutrons.^[18] The attempted reactions used light titanium nuclei aimed at targets consisting of lead isotopes. The first such reaction was studied in 1974 by the team at Dubna:

20882Pb + 5022Ti → 258-x104Rf + x n (x=1, 2, or 3)

The measurement of a spontaneous fission activity was assigned to ²⁵⁶Rf.^[19] Later, in 1985, this reaction was further studied in 1985 by the GSI team who measured the decay properties of the isotopes ²⁵⁷Rf and ²⁵⁶Rf.^[20] After an upgrade of the facilities, the team as GSI repeated the reaction in 1994 with much higher sensitivity and detected some 1100 atoms of ²⁵⁷Rf and 1900 atoms of ²⁵⁶Rf along with ²⁵⁵Rf, and isomeric levels for ²⁵⁷Rf and ²⁵⁵Rf.^[21] The reaction was further studied in 2002 by scientists at the Argonne University in Illinois,^[22] while in 2007, the Lawrence Berkeley National Laboratory (LBNL) reported further isomers of ²⁵⁶Rf.^[23]

Researchers at Dubna also investigated the reaction of ²⁰⁷Pb with ⁵⁰Ti to produce ²⁵⁵Rf in 1974. The reaction was further studied in 1985 by the GSI team who measured the decay properties of the isotope ²⁵⁵Rf, and in 2000 they published the first decay level scheme for the isotope.^[24] In a 1994 study by GSI with the ²⁰⁶Pb isotope instead, ²⁵⁵Rf and 144 atoms of the new isotope ²⁵⁴Rf were detected. The latter decayed by spontaneous fission.^{[21] 253}Rf was similarly detected that year when ²⁰⁴Pb was used instead, and similarly decayed by spontaneous fission.^[21]

Decay studies

No isotopes with an atomic mass of 262 have been synthesized by direct fusion reactions. Decay reactions of elements with higher atomic number have generated many of these isotopes, together with some heavier ones.

Investigations on the synthesis of the isotope ²⁶³Rf were done in 1999 at the University of Bern, Switzerland on the reaction:

24896Cm + 2210Ne → 266-x104Rf + α + x n (x=3?).

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.^[25]

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.

Isotopes

Rutherfordium does not have any stable or naturally occurring isotopes. However, a several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms (see next section), or by observing the decay of heavier elements that rutherfordium. At least 15 different isotopes have been reported so far ranging in their atomic mass from 253 up to 268.^[2]

Currently suggested decay level scheme for ²⁵⁷Rf^g,m from the study performed in 2004 by Hessberger et al. at GSI

Currently suggested decay level scheme for ²⁵⁵Rf from the study reported in 2007 by Hessberger et al. at GSI^[26]

The lighter isotopes usually have shorter half lives, e.g., under 50 μs for ²⁵³Rf and ²⁵⁴Rf. ²⁵⁶Rf, ²⁵⁸Rf, ²⁶⁰Rf are more stable at around 10 ms, ²⁵⁵Rf, ²⁵⁷Rf, ²⁵⁹Rf, and ²⁶²Rf live between 1 and 5 seconds, and ²⁶¹Rf and ²⁶³Rf are more stable, at around 1 and 10 min, respectively. The heaviest known isotopes are the most stable, with ²⁶⁵Rf having a reported half-life of about 13 h. Isotopes ²⁶⁴Rf, ²⁶⁶Rf and ²⁶⁸Rf have also been observed to have long half-lives of 1 h, 10 h and 6 h, respectively, but these have only been measured indirectly through systematic studies.^[2]

Other

The lightest isotopes were synthesized by direct fusion between two lighter nuclei. The heaviest isotope produced this way is ²⁶²Rf, meaning that the remaining ones have only been observed as decay products of elements with larger atomic numbers.

Unconfirmed and retracted isotopes

²⁶⁸Rf

In the synthesis of ununpentium, the isotope ²⁸⁸115 decayed to ²⁶⁸Db which undergoes spontaneous fission with a half-life of 29 hours. Given that the electron capture of ²⁶⁸Db cannot be detected, these SF events may be due to the SF of ²⁶⁸Rf, in which case the half-life of this isotope cannot be extracted.^{[note 3]}

²⁶⁶Rf

In the synthesis of ununtrium, the isotope ²⁸²113 decayed to ²⁶⁶Db which undergoes spontaneous fission with a half-life of 22 minutes. Given that the electron capture of ²⁶⁶Db cannot be detected, these SF events may be due to the SF of ²⁶⁶Rf, in which case the half-life of this isotope cannot be extracted. ^{[note 1]}

²⁶⁵Rf

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.^{[note 4]}

Nuclear isomerism

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 of 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.^[27]

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. ^[28] 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 complex spectrum with 15 alpha lines. A level structure diagram was calculated for both isomers.^[29]

Chemical properties

Group 4 membership

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. 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.^[9]

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.^[9]

In the aqueous phase, the Rf⁴⁺ ion hydrolyzes 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.^[9]

Experimental chemistry

Gas phase

The tetrahedral molecule RfCl₄

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.^[9][30]

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).^[9][31]

Aqueous phase

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.^[9]

The most conclusive aqueous chemistry studies of rutherfordium have been performed by the Japanese team at JAERI using the radioisotope ^261mRf. Extraction experiments from hydrochloric acid solutions using isotopes of rutherfordium, hafnium, zirconium and thorium have proved a non-actinide behavior. 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. ^{[9] [32]}

^261mRf⁴⁺ + 6 Cl⁻ → [^261mRfCl₆]²⁻

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:^[9]

^261mRf⁴⁺ + 6 F⁻ → [^261mRfF₆]²⁻

Notes

References

External links

• WebElements.com - Rutherfordium