Copernicium

Copernicium
112Cn
Hg ↑ Cn ↓ (Uhq) Periodic table roentgenium ← copernicium → ununtrium
Appearance
unknown
General properties
Name, symbol, number	copernicium, Cn, 112
Pronunciation	/ k oʊ p ər ˈ n ɪ s i ə m / koe-pər-NIS-ee-əm
Metallic category	transition metal
Group, period, block	12, 7, d
Standard atomic weight	[285]
Electron configuration	[Rn] 5f14 6d10 7s2 (predicted) 2, 8, 18, 32, 32, 18, 2 (predicted)
History
Discovery	Gesellschaft für Schwerionenforschung (1996)
Physical properties
Phase	unknown
Density (near r.t.)	23.7 (predicted) g·cm−3
Atomic properties
Oxidation states	4, 2, 0 (predicted)
Ionization energies ( more)	1st: 1154.9 (estimated) kJ·mol−1
	2nd: 2170.0 (estimated) kJ·mol−1
	3rd: 3164.7 (estimated) kJ·mol−1
Atomic radius	110 (predicted) pm
Covalent radius	122 (predicted) pm
Miscellanea
CAS registry number	54084-26-3
Most stable isotopes
Main article: Isotopes of copernicium
iso NA half-life DM DE ( MeV) DP 285Cn syn 29 s α 9.15,9.03? 281Ds 285mCn ? syn 8.9 min α 8.63 281mDs ? 283Cn syn 4 s 90% α 9.53,9.32,8.94 279Ds 10% SF 283mCn ?? syn ~7.0 min SF only isotopes with half-lives over 1 second are included here

Copernicium is a chemical element with symbol Cn and atomic number 112. It is an extremely radioactive synthetic element that can only be created in a laboratory. The most stable known isotope, copernicium-285, has a half-life of approximately 29 seconds, but it is possible that this copernicium isotope may have a nuclear isomer with a longer half-life, 8.9 min. Copernicium was first created in 1996 by the GSI Helmholtz Centre for Heavy Ion Research near Darmstadt, Germany. It is named after the astronomer Nicolaus Copernicus.

In the periodic table of the elements, it is a d-block transactinide element. During reactions with gold, it has been shown to be an extremely volatile metal and a group 12 element, and it may even be a gas at standard temperature and pressure. Copernicium is calculated to have several properties that differ between it and its lighter homologues, zinc, cadmium and mercury; the most notable of them is withdrawing two 6d-electrons before 7s ones due to relativistic effects, which confirm copernicium as an undisputed transition metal. Copernicium is also calculated to show a predominance of the oxidation state +4, while mercury shows it in only one compound at extreme conditions and zinc and cadmium do not show it at all. It has also been predicted to be more difficult to oxidise copernicium from its neutral state than the other group 12 elements.

In total, approximately 75 atoms of copernicium have been detected using various nuclear reactions.

History

Official discovery

Copernicium was first created on February 9, 1996, at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, by Sigurd Hofmann, Victor Ninov et al. This element was created by firing accelerated zinc-70 nuclei at a target made of lead-208 nuclei in a heavy ion accelerator. A single atom (the second was subsequently dismissed) of copernicium was produced with a mass number of 277.

208
82Pb + 70
30Zn → 278
112Cn → 277
112Cn + 1
0n

In May 2000, the GSI successfully repeated the experiment to synthesize a further atom of copernicium-277. This reaction was repeated at RIKEN using the Search for a Super-Heavy Element Using a Gas-Filled Recoil Separator set-up in 2004 to synthesize two further atoms and confirm the decay data reported by the GSI team.

The IUPAC/IUPAP Joint Working Party (JWP) assessed the claim of discovery by the GSI team in 2001 and 2003. In both cases, they found that there was insufficient evidence to support their claim. This was primarily related to the contradicting decay data for the known nuclide rutherfordium-261. However, between 2001 and 2005, the GSI team studied the reaction ²⁴⁸Cm(²⁶Mg,5n)²⁶⁹Hs, and were able to confirm the decay data for hassium-269 and rutherfordium-261. It was found that the existing data on rutherfordium-261 was for an isomer, now designated rutherfordium-261a.

In May 2009, the JWP reported on the claims of discovery of element 112 again and officially recognized the GSI team as the discoverers of element 112. This decision was based on the confirmation of the decay properties of daughter nuclei as well as the confirmatory experiments at RIKEN.

Naming

After acknowledging their discovery, the IUPAC asked the discovery team at GSI to suggest a permanent name for element 112. On 14 July 2009, they proposed copernicium with the element symbol Cp, after Nicolaus Copernicus "to honour an outstanding scientist, who changed our view of the world." IUPAC delayed the official recognition of the name, pending the results of a six-month discussion period among the scientific community.

However, it was pointed out that the symbol Cp was previously associated with the name cassiopeium (cassiopium), now known as lutetium (Lu). Furthermore, the symbol Cp is also used in organometallic chemistry to denote the cyclopentadienyl ligand. For this reason, the IUPAC disallowed the use of Cp as a future symbol, prompting the GSI team to put forward the symbol Cn as an alternative. On 19 February 2010, the 537th anniversary of Copernicus' birth, IUPAC officially accepted the proposed name and symbol.

Nucleosynthesis

Super-heavy elements such as copernicium are produced by bombarding lighter elements in particle accelerators that induces fusion reactions. Whereas most of the isotopes of copernicium can be synthesized directly this way, some heavier ones have only been observed as decay products of elements with higher atomic numbers.

Depending on the energies involved, the former are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets such as actinides, giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).

Cold fusion

The first cold fusion reaction to produce copernicium was performed by GSI in 1996, who reported the detection of two decay chains of copernicium-277.

208
82Pb + 70
30Zn → 277
112Cn + n

In a review of the data in 2000, the first decay chain was retracted. In a repeat of the reaction in 2000 they were able to synthesize a further atom. They attempted to measure the 1n excitation function in 2002 but suffered from a failure of the zinc-70 beam. The unofficial discovery of copernicium-277 was confirmed in 2004 at RIKEN, where researchers detected a further two atoms of the isotope and were able to confirm the decay data for the entire chain.

After the successful synthesis of copernicium-277, the GSI team performed a reaction using a ⁶⁸Zn projectile in 1997 in an effort to study the effect of isospin (neutron richness) on the chemical yield.

208
82Pb + 68
30Zn → 276-x
112Cn + x n

The experiment was initiated after the discovery of a yield enhancement during the synthesis of darmstadtium isotopes using nickel-62 and nickel-64 ions. No decay chains of copernicium-275 were detected leading to a cross section limit of 1.2  picobarns (pb). However, the revision of the yield for the zinc-70 reaction to 0.5 pb does not rule out a similar yield for this reaction.

In 1990, after some early indications for the formation of isotopes of copernicium in the irradiation of a tungsten target with multi-GeV protons, a collaboration between GSI and the Hebrew University studied the foregoing reaction.

184
74W + 88
38Sr → 272-x
112Cn + x n

They were able to detect some spontaneous fission (SF) activity and a 12.5 MeV alpha decay, both of which they tentatively assigned to the radiative capture product copernicium-272 or the 1n evaporation residue copernicium-271. Both the TWG and JWP have concluded that a lot more research is required to confirm these conclusions.

Hot fusion

In 1998, the team at the Flerov Laboratory of Nuclear Research (FLNR) in Dubna, Russia began a research program using calcium-48 nuclei in "warm" fusion reactions leading to super-heavy elements. In March 1998, they claimed to have synthesized two atoms of the element in the following reaction.

238
92U + 48
20Ca → 286-x
112Cn + x n (x=3,4)

The product, copernicium-283, had a claimed half-life of 5 minutes, decaying by spontaneous fission.

The long half-life of the product initiated first chemical experiments on the gas phase atomic chemistry of copernicium. In 2000, Yuri Yukashev in Dubna repeated the experiment but was unable to observe any spontaneous fission with half-life of 5 minutes. The experiment was repeated in 2001 and an accumulation of eight fragments resulting from spontaneous fission were found in the low-temperature section, indicating that copernicium had radon-like properties. However, there is now some serious doubt about the origin of these results. To confirm the synthesis, the reaction was successfully repeated by the same team in January 2003, confirming the decay mode and half-life. They were also able to calculate an estimate of the mass of the spontaneous fission activity to ~285, lending support to the assignment.

The team at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, United States entered the debate and performed the reaction in 2002. They were unable to detect any spontaneous fission and calculated a cross section limit of 1.6 pb for the detection of a single event.

The reaction was repeated in 2003–2004 by the team at Dubna using a slightly different set-up, the Dubna Gas-Filled Recoil Separator (DGFRS). This time, copernicium-283 was found to decay by emission of a 9.53 MeV alpha-particle with a half-life of 4 seconds. copernicium-282 was also observed in the 4n channel (emitting 4 neutrons).

In 2003, the team at GSI entered the debate and performed a search for the five-minute SF activity in chemical experiments. Like the Dubna team, they were able to detect seven SF fragments in the low temperature section. However, these SF events were uncorrelated, suggesting they were not from actual direct SF of copernicium nuclei and raised doubts about the original indications for radon-like properties. After the announcement from Dubna of different decay properties for copernicium-283, the GSI team repeated the experiment in September 2004. They were unable to detect any SF events and calculated a cross section limit of ~1.6 pb for the detection of one event, not in contradiction with the reported 2.5 pb yield by Dubna team.

In May 2005, the GSI performed a physical experiment and identified a single atom of ²⁸³Cn decaying by SF with a short half-time suggesting a previously unknown SF branch. However, initial work by Dubna team had detected several direct SF events but had assumed that the parent alpha decay had been missed. These results indicated that this was not the case.

The new decay data on copernicium-283 were confirmed in 2006 by a joint PSI–FLNR experiment aimed at probing the chemical properties of copernicium. Two atoms of copernicium-283 were observed in the decay of the parent flerovium-287 nuclei. The experiment indicated that contrary to previous experiments, copernicium behaves as a typical member of group 12, demonstrating properties of a volatile metal.

Finally, the team at GSI successfully repeated their physical experiment in January 2007, and detected three atoms of copernicium-283, confirming both the alpha and SF decay modes.

As such, the 5 minutes SF activity is still unconfirmed and unidentified. It is possible that it refers to an isomer, namely copernicium-283b, whose yield is dependent upon the exact production methods.

233
92U + 48
20Ca → 281-x
112Cn + x n

The team at FLNR studied this reaction in 2004. They were unable to detect any atoms of copernicium and calculated a cross section limit of 0.6 pb. The team concluded that this indicated that the neutron mass number for the compound nucleus had an effect on the yield of evaporation residues.

Decay products

Copernicium has been observed as decay products of flerovium. Flerovium currently has five known isotopes, all of which have been shown to undergo alpha decays to become copernicium nuclei, with mass numbers between 281 and 285. Copernicium isotopes with mass numbers 281, 284 and 285 to date have only been produced by flerovium nuclei decay. Parent flerovium nuclei can be themselves decay products of livermorium or ununoctium. To date, no other elements have been known to decay to copernicium.

For example, in May 2006, the Dubna team ( JINR) identified copernicium-282 as a final product in the decay of ununoctium via the alpha decay sequence. It was found that the final nucleus undergoes spontaneous fission.

294
118Uuo → 290
116Lv + 4
2He

290
116Lv → 286
114Fl + 4
2He

286
114Fl → 282
112Cn + 4
2He

In the claimed synthesis of ununoctium-293 in 1999, copernicium-281 was identified as decaying by emission of a 10.68 MeV alpha particle with half-life 0.90 ms. The claim was retracted in 2001. This isotope was finally created in 2010 and its decay properties contradicted the previous data.

Isotopes

Copernicium 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. Six different isotopes have been reported with atomic masses from 281 to 285, and 277, two of which, copernicium-283 and copernicium-285, have known metastable states. Most of these decay predominantly through alpha decay, but some undergo spontaneous fission.

The isotope copernicium-283 was instrumental in the confirmation of the elements flerovium and livermorium.

Half-lives

All copernicium isotopes are extremely unstable and radioactive; in general, heavier isotopes are more stable than the lighter. The most stable isotope, copernicium-285, has a half-life of 29 seconds, although it is suspected that this isotope has an isomer with a half-life of 8.9 minutes, and copernicium-283 may have an isomer with a half-life of about 5 minutes. Other isotopes have half-lives shorter than 0.1 seconds. Copernicium-281 and copernicium-284 have half-life of 97 ms, and the other two isotopes have half-lives slightly under one millisecond.

The lightest isotopes were synthesized by direct fusion between two lighter nuclei and as decay products (except for copernicium-277, which is known to be a decay product), while the heavier isotopes are only known to be produced by decay of heavier nuclei. The heaviest isotope produced by direct fusion is copernicium-283; the two heavier isotopes, copernicium-284 and copernicium-285 have only been observed as decay products of elements with larger atomic numbers. In 1999, American scientists at the University of California, Berkeley, announced that they had succeeded in synthesizing three atoms of ²⁹³118. These parent nuclei were reported to have successively emitted three alpha particles to form copernicium-281 nuclei, which were claimed to have undergone an alpha decay, emitting an alpha particle with decay energy of 10.68 MeV and half-life 0.90 ms, but their claim was retracted in 2001. The isotope, however, was produced in 2010 by the same team. The new data contradicted the previous (fabricated) data.

Nuclear isomerism

First experiments on the synthesis of ²⁸³Cn produced a SF activity with half-life ~5 min. This activity was also observed from the alpha decay of flerovium-287. The decay mode and half-life were also confirmed in a repetition of the first experiment. Later, copernicium-283 was observed to undergo 9.52 MeV alpha decay and SF with a half-life of 3.9 s. It has also been found that alpha decay of copernicium-283 leads to different excited states of darmstadtium-279. These results suggest the assignment of the two activities to two different isomeric levels in copernicium-283, creating copernicium-283a and copernicium-283b.

Copernicium-285 has only been observed as a decay product of flerovium-289 and livermorium-293; during the first recorded synthesis of flerovium, one flerovium-289 was created, which alpha decayed to copernicium-285, which itself emitted an alpha particle in 29 seconds, releasing 9.15 or 9.03 MeV. However, in the first experiment to successfully synthesize livermorium, when livermorium-293 was created, it was shown that the created nuclide alpha decayed to flerovium-289, decay data for which differed from the known values significantly. Although unconfirmed, it is highly possible that this is associated with an isomer. The resulting nuclide decayed to copernicium-285, which emitted an alpha particle with a half-life of around 10 minutes, releasing 8.586 MeV. Similar to its parent, it is believed to be a nuclear isomer, copernicium-285b.

Predicted properties

Chemical

Copernicium is the last member of the 6d series of transition metals and the heaviest group 12 element in the periodic table, below zinc, cadmium and mercury. It is predicted to differ significantly from lighter group 12 elements. Due to stabilization of 7s electronic orbitals and destabilization of 6d ones caused by relativistic effects, Cn²⁺ is likely to have a [Rn]5f¹⁴6d⁸7s² electronic configuration, breaking 6d orbitals before 7s one, unlike its homologues. The fact that the 6d electrons participate readily in chemical bonding mean that copernicium should behave more like a transition metal than its lighter homologues, especially in the +4 oxidation state. In water solutions, copernicium is likely to form +2 and +4 oxidation states, with the latter one being more stable. Among lighter group 12 members, for which the +2 oxidation state is the most common, only mercury can show +4 oxidation state, but it is highly uncommon, existing at only one compound ( mercury(IV) fluoride, HgF₄) at extreme conditions. The analogous compound for copernicium, copernicium(IV) fluoride (CnF₄), is predicted to be more stable. The diatomic ion Hg2+
2, featuring mercury in +1 oxidation state is well-known, but the Cn2+
2 ion is predicted to be unstable or even non-existent. Oxidation of copernicium from its neutral state is also likely to be more difficult than those of previous group 12 members. Copernicium(II) fluoride, CnF₂, should be more unstable than the analogous mercury compound, mercury(II) fluoride (HgF₂), and may even decompose spontaneously into its constituent elements. In polar solvents, copernicium is predicted to preferentially form the CnF−
5 and CnF−
3 anions rather than the analogous neutral fluorides (CnF₄ and CnF₂, respectively), although the analogous bromide or iodide ions may be more stable towards hydrolysis in aqueous solution. The anions CnCl2−
4 and CnBr2−
4 should also be able to exist in aqueous solution.

The valence s- subshells of the group 12 elements and period 7 elements are expected to be relativistically contracted most strongly at copernicium. This and the closed-shell configuration of copernicium result in it probably being a very noble metal. Its metallic bonds should also be very weak, possibly making it extremely volatile, like the noble gases, and potentially making it gaseous at room temperature. However, it should be able to form metal–metal bonds with copper, palladium, platinum, silver, and gold; these bonds are predicted to be only about 15–20  kJ/mol weaker than the analogous bonds with mercury.

Physical and atomic

Copernicium should be a very heavy metal with a density of around 23.7 g/cm³ in the solid state; in comparison, the most dense known element that has had its density measured, osmium, has a density of only 22.61 g/cm³. This results from copernicium's high atomic weight, the lanthanide and actinide contractions, and relativistic effects, although production of enough copernicium to measure this quantity would be impractical, and the sample would quickly decay. However, some calculations predict copernicium to be a gas at room temperature, the first gaseous metal in the periodic table (the second being flerovium), due to the closed-shell electron configurations of copernicium and flerovium. The atomic radius of copernicium is expected to be around 110 pm. Due to the relativistic stabilization of the 7s orbital and destabilization of the 6d orbital, the Cn⁺ and Cn²⁺ ions are predicted to give up 6d electrons instead of 7s electrons, which is the opposite of the behaviour of its lighter homologues.

Experimental atomic gas phase chemistry

Copernicium has the ground state electron configuration [Rn]5f¹⁴6d¹⁰7s² and thus should belong to group 12 of the periodic table, according to the Aufbau principle. As such, it should behave as the heavier homologue of mercury and form strong binary compounds with noble metals like gold. Experiments probing the reactivity of copernicium have focused on the adsorption of atoms of element 112 onto a gold surface held at varying temperatures, in order to calculate an adsorption enthalpy. Owing to relativistic stabilization of the 7s electrons, copernicium shows radon-like properties. Experiments were performed with the simultaneous formation of mercury and radon radioisotopes, allowing a comparison of adsorption characteristics.

The first experiments were conducted using the ²³⁸U(⁴⁸Ca,3n)²⁸³Cn reaction. Detection was by spontaneous fission of the claimed parent isotope with half-life of 5 minutes. Analysis of the data indicated that copernicium was more volatile than mercury and had noble gas properties. However, the confusion regarding the synthesis of copernicium-283 has cast some doubt on these experimental results. Given this uncertainty, between April–May 2006 at the JINR, a FLNR–PSI team conducted experiments probing the synthesis of this isotope as a daughter in the nuclear reaction ²⁴²Pu(⁴⁸Ca,3n)²⁸⁷Fl. In this experiment, two atoms of copernicium-283 were unambiguously identified and the adsorption properties indicated that copernicium is a more volatile homologue of mercury, due to formation of a weak metal-metal bond with gold, placing it firmly in group 12.

In April 2007, this experiment was repeated and a further three atoms of copernicium-283 were positively identified. The adsorption property was confirmed and indicated that copernicium has adsorption properties completely in agreement with being the heaviest member of group 12.