Rutherfordium

Rutherfordium (pronEng|ˌrʌðɚˈfɔrdiəm) is a chemical element in the periodic table that has the symbol Rf and atomic number 104. This is a radioactive synthetic element whose most stable known isotope is ²⁶⁷Rf with a half-life of approximately 1.3 hours. Chemistry experiments have confirmed that rutherfordium behaves as the heavier homologue to hafnium in group 4 (see below).

Official discovery

Element 104 was reportedly first detected in 1966 at the Joint Institute of Nuclear Research at Dubna (U.S.S.R.). 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: [http://www.iupac.org/publications/pac/1993/pdf/6508x1757.pdf "DISCOVERY OF THE TRANSFERMIUM ELEMENTS"] , IUPAC/IUPAP Technial Report, "Pure & Appl. Chem.", Vol. 65, No. 8, pp. 1757-1814,1993. Retrieved on 2008-03-04]

:$$, ^{242}_{94}mathrm{Pu} + , ^{22}_{10}mathrm{Ne} o , ^{264-x}_{104}mathrm{Rf} o ,^{264-x}_{104}mathrm{RfCl}_{4}

The Russian scientists proposed the name "Kurchatovium" for the new element.

In 1969 researchers at the University of California, Berkeley conclusively synthesized the element by bombarding a Cf-249 target with carbon-12 ions and measured the alpha decay of ²⁵⁷104, correlated with the daughter decay of ²⁵³102. [http://prola.aps.org/abstract/PRL/v22/i24/p1317_1 "Positive Identification of Two Alpha-Particle-Emitting Isotopes of Element 104"] , Ghiorso et al., "Phys. Rev. Lett. 22", 1317-1320 (1969). Retrieved on 2008-03-04]

:$$,^{249}_{98}mathrm{Cf} + ,^{12}_{6}mathrm{C} o ,^{257}_{104}mathrm{Rf} + 4 ,^{1}_{0}mathrm{n}

The American scientists proposed the name "Rutherfordium" for the new element.

The American synthesis was independently confirmed in 1973 and secured the identification of element 104 as the parent by the observation of K X-rays in the elemental signature of the daughter ²⁵³No. [http://prola.aps.org/abstract/PRL/v31/i10/p647_1 "X-Ray Identification of Element 104"] , Bemis et al., "Phys. Rev. Lett. 31", 647-650 (1973). Retrieved on 2008-03-04]

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 which the Russian team do 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. [ [http://www.iupac.org/publications/pac/1993/pdf/6508x1815.pdf "Responses on the Report DISCOVERY OF THE TRANSFERMIUM ELEMENTS"] , "Pure & Appl. Chem.",Vol. 65, No. 8, pp. 1815-1824, 1993. Retrieved on 2008-04-03] It should be said that IUPAC finally used the name suggested by the American team ("rutherfordium") which may in some way reflect a change of opinion. [http://www.iupac.org/publications/pac/1997/pdf/6912x2471.pdf "NAMES AND SYMBOLS OF TRANSFERMIUM ELEMENTS"] , IUPAC, "Pure & Appl. Chem.", Vol. 69, No. 12, pp. 2471-2473, 1997. Retrieved on 2008-04-03]

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" (pronEng|ˌkɝtʃəˈtoʊviəm, Ku, in honor of Igor Vasilevich 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" (pronounced IPA|/ˌjuːnɪlˈkwɒdiəm/ or IPA|/ˌʌnɪlˈkwɒdiəm/, 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 recognised 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".

Electronic structure

Rutherfordium is element 104 in the Periodic Table. The two forms of the projected electronic structure are:

Bohr model: 2, 8, 18, 32, 32, 10, 2

Quantum mechanical model: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p⁶7s²5f¹⁴6d²

Extrapolated chemical properties of eka-hafnium/dvi-zirconium

Oxidation states

Element 104 is projected to be 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.The IV oxidation state is the only stable state for the latter two elements and therefore rutherfordium should also portray a stable +4 state.

Chemistry

In an analogous manner to zirconium and hafnium, rutherfordium is projected to form a very stable, high melting point oxide, RfO₂. It should also react with halogens to form tetrahalides, RfX₄, which hydrolyse on contact with water to form oxyhalides RfOX₂. The tetrahalides should be volatile solids existing as monomeric tetrahedral molecules in the vapour phase.In the aqueous phase, the Rf⁴⁺ ion should hydrolyse less than titanium(IV) and to a similar extent to zirconium and hafnium, thus leading to the rutherfordyl oxyion, RfO₂²⁺. Treatment of the halides with halide ions promotes the formation of complex ions. The use of chloride and bromide ion should form the hexahalide complexes RfCl₆^2- and RfBr₆^2-. For the fluoride complexes, zirconium and hafnium tend to form hepta- and octa- complexes. Thus, for the larger rutherfordium ion, the complexes RfF₆^2-, RfF₇^3- and RfF₈^4- are possible.

Experimental chemistry

Gas phase chemistry

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 from 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). [http://lch.web.psi.ch/pdf/TexasA&M/TexasA&M.pdf (Gas Chem 2007 Review)]

Aqueous phase chemistry

Rutherfordium is expected to have the electron configuration [Rf] 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 tean 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 behaviour. 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. [http://wwwsoc.nii.ac.jp/jnrs/paper/JN62/jn6202.pdf (Rf aqueous chem JAERI)]

:$$,^{261m}mathrm{Rf} + 6mathrm{Cl}^{-}, o [,^{261m}mathrm{RfCl}_{6}] ^{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:

:$$,^{261m}mathrm{Rf} + 6mathrm{F}^{-}, o [,^{261m}mathrm{RfF}_{6}] ^{2-}

ummary of compounds and complex ions

Isomerism in rutherfordium nuclides

²⁶³Rf

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. It is reasonable to tentatively assign the long life to a meta-stable state, namely ^263mRf, and the shorter life to the ground state, namely ^263gRf. Further studies are required to allow a definite assignment.

²⁶¹Rf

Early research on the synthesis of rutherfordium isotopes utilised 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 element 112 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 element 112. However, it is now understood that the first nucleus belongs to an isomeric meta-stable state, namely ^261mRf and the latter to the ground state isomer, namely ^261gRf. [ [http://lch.web.psi.ch/pdf/anrep01/B-02heavies.pdf "EVIDENCE FOR ISOMERIC STATES IN ²⁶¹Rf"] , Dressler et al., "PSI Annual Report 2001". Retrieved on 2008-01-29] The discovery and confirmation of ^261gRf provided proof for the discovery of ununbium in 1996.

²⁵⁷Rf

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.

pectroscopic decay level schemes for rutherfordium isotopes

²⁵⁷Rf

²⁵⁵Rf

Unconfirmed isotopes

²⁶⁸Rf

In the synthesis of ununpentium, the isotope ²⁸⁸115 has been observed to decay 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 in fact be due to the SF of ²⁶⁸Rf, in which case the half-life of this isotope cannot be extracted.

²⁶⁶Rf

In the synthesis of ununtrium, the isotope ²⁸²113 has been observed to decay 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 in fact be due to the SF of ²⁶⁶Rf, in which case the half-life of this isotope cannot be extracted.

Retracted isotopes

²⁶⁵Rf

In 1999, American scientists at the University of California, Berkeley, announced that they has 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. [see ununoctium]

^255mRf

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.

Chemical yields of isotopes

Cold fusion

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.

Hot fusion

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.

Future experiments

The team at GSI are planning to perform first detailed spectroscopic studies on the isotope ²⁵⁹Rf. It will be produced in the new reaction:

:$$,^{238}_{92}mathrm{U} + ,^{24}_{12}mathrm{Mg} o ,^{259}_{104}mathrm{Rf} + 3 ,^{1}_{0}mathrm{n}

References

External links

* [http://www.webelements.com/webelements/elements/text/Rf/index.html WebElements.com - Rutherfordium]