Rutherfordium

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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.[11] 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.
Introduction

A graphic depiction of a nuclear fusion reaction

External videos
[12]

The heaviest[a] atomic nuclei are created in nuclear reactions that combine two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[18] The material made of the heavier nuclei is made into a target, which is then bombarded by the beam of lighter nuclei. Two nuclei can fuse into one only if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to electrostatic repulsion. The strong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly accelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[19] Coming close alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[19][20] If fusion does occur, the temporary merger—termed a compound nucleus—is an excited state. To lose its excitation energy and reach a more stable state, a compound nucleus either fissions or ejects one or several neutrons,[c] which carry away the energy. This occurs in approximately 10−16 seconds after the initial collision.[21][d]

The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam.[24] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[e] and transferred to a surface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival.[24] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[27] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[24]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, their influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, as it has unlimited range.[28] Nuclei of the heaviest elements are thus theoretically predicted[29] and have so far been observed[30] to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[f] these modes are predominant for nuclei of superheavy elements. Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be determined arithmetically.[g] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[h]

The information available to physicists aiming to synthesize one of the heaviest elements is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[i]
History
Discovery

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 and separated 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. Though a half-life was not accurately determined, later calculations indicated that the product was most likely rutherfordium-259 (259Rf in standard notation):[42]

242
94Pu
+ 22
10Ne
→ 264−x
104Rf
→ 264−x
104Rf
Cl4

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 257Rf, correlated with the daughter decay of nobelium-253:[43]

249
98Cf
+ 12
6C
→ 257
104Rf
+ 4
n

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 257Rf decay product, nobelium-253.[44]
Naming controversy
Main article: Transfermium Wars
Element 104 was eventually named after Ernest Rutherford

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.[45] 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.[42]

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.[46] The IUPAC finally used the name suggested by the American team (rutherfordium).[47]

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.[48] 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.[49] In 1997, IUPAC renamed elements 104 to 109, and gave element 104 the current name rutherfordium. The name dubnium was given to element 105 at the same time.[47]
Isotopes
Isotope half-lives and discovery years Isotope
Half-life
[8] Decay
mode[8] Discovery
year Reaction
253Rf 48 μs α, SF 1994 204Pb(50Ti,n)[50]
254Rf 23 μs SF 1994 206Pb(50Ti,2n)[50]
255Rf 2.3 s ε?, α, SF 1974 207Pb(50Ti,2n)[51]
256Rf 6.4 ms α, SF 1974 208Pb(50Ti,2n)[51]
257Rf 4.7 s ε, α, SF 1969 249Cf(12C,4n)[43]
257mRf 4.1 s ε, α, SF 1969 249Cf(12C,4n)[43]
258Rf 14.7 ms α, SF 1969 249Cf(13C,4n)[43]
259Rf 3.2 s α, SF 1969 249Cf(13C,3n)[43]
259mRf 2.5 s ε 1969 249Cf(13C,3n)[43]
260Rf 21 ms α, SF 1969 248Cm(16O,4n)[42]
261Rf 78 s α, SF 1970 248Cm(18O,5n)[52]
261mRf 4 s ε, α, SF 2001 244Pu(22Ne,5n)[53]
262Rf 2.3 s α, SF 1996 244Pu(22Ne,4n)[54]
263Rf 15 min α, SF 1999 263Db(
e−
,
ν
e)[55]
263mRf ? 8 s α, SF 1999 263Db(
e−
,
ν
e)[55]
265Rf 1.1 min[9] SF 2010 269Sg(—,α)[56]
266Rf 23 s? SF 2007? 266Db(
e−
,
ν
e)?[57][58]
267Rf 48 min[10] SF 2004 271Sg(—,α)[59]
268Rf 1.4 s? SF 2004? 268Db(
e−
,
ν
e)?[58][60]
270Rf 20 ms?[61] SF 2010? 270Db(
e−
,
ν
e)?[62]
Main article: Isotopes of rutherfordium

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. Sixteen different isotopes have been reported with atomic masses from 253 to 270 (with the exceptions of 264 and 269). Most of these decay predominantly through spontaneous fission pathways.[8][63]
Stability and half-lives

Out of isotopes whose half-lives are known, the lighter isotopes usually have shorter half-lives; half-lives of under 50 μs for 253Rf and 254Rf were observed. 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 heaviest isotopes are the most stable, with 267Rf having a measured half-life of about 48 minutes.[10]

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.[62] 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.[64] These parent nuclei were reported to have successively emitted seven alpha particles to form 265Rf nuclei, but their claim was retracted in 2001.[65] This isotope was later discovered in 2010 as the final product in the decay chain of 285Fl.[9][56]
Predicted properties

Very few properties of rutherfordium or its compounds have been measured; this is due to its extremely limited and expensive production[18] 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

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.[55][66] 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.[66] In addition, rutherfordium is also expected to be able to form a less stable +3 state.[2] The standard reduction potential of the Rf4+/Rf couple is predicted to be higher than −1.7 V.[5]

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.[2][66] Later it was shown in ab initio calculations with the high level of accuracy[67][68][69] 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.[66]

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 RfCl2−
6 and RfBr2−
6. For the fluoride complexes, zirconium and hafnium tend to form hepta- and octa- complexes. Thus, for the larger rutherfordium ion, the complexes RfF2−
6, RfF3−
7 and RfF4−
8 are possible.[66]
Physical and atomic

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.[6] It should be a metal with density ~17 g/cm3.[3][4] 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.[2] 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.[70]
Experimental chemistry
Gas phase
The tetrahedral structure of the RfCl4 molecule

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,[66] though the long-lived isotope 267Rf (produced in the decay chain of 291Lv, 287Fl, and 283Cn) may be advantageous for future experiments.[71] 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.[66][72][73] Rutherfordium(IV) chloride is more volatile than its lighter homologue hafnium(IV) chloride (HfCl4) because its bonds are more covalent.[2]

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 RfCl
4 when potassium chloride is provided as the solid phase instead of gas, highly indicative of the formation of nonvolatile K
2RfCl
6 mixed salt.[55][66][74]
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.[66]

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.[66][75]

261m
Rf4+
+ 6 Cl−
→ [261mRfCl
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:[66]

261m
Rf4+
+ 6 F−
→ [261mRfF
6]2−

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.[76]

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.[77]
Notes

In nuclear physics, an element is called heavy if its atomic number is high; lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than 103 (although there are other definitions, such as atomic number greater than 100[13] or 112;[14] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series).[15] Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language—isotopes of high mass (for the given element) and nuclei of high mass, respectively.
In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric 136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb.[16] In comparison, the reaction that resulted in hassium discovery, 208Pb + 58Fe, had a cross section of ~20 pb (more specifically, 19+19
−11 pb), as estimated by the discoverers.[17]
The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a gamma ray.[21]
The definition by the IUPAC/IUPAP Joint Working Party states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire its outer electrons and thus display its chemical properties.[22] This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[23]
This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle.[25] Such separation can also be aided by a time-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[26]
Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[31]
Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for heaviest nuclei.[32] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[33] Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).[34]
Spontaneous fission was discovered by Soviet physicist Georgy Flerov,[35] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[36] In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles.[23] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[35]

For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[37] There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers, nobelium. It was later shown that the identification was incorrect.[38] The following year, LBNL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[38] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[39] the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty").[40] The name "nobelium" remained unchanged on account of its widespread usage.[41]

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