Flerovium

Flerovium is a synthetic chemical element; it has symbol Fl and atomic number 114. It is an extremely radioactive, superheavy element, named after the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna, Russia, where the element was discovered in 1999. The lab's name, in turn, honours Russian physicist Georgy Flyorov. IUPAC adopted the name on 30 May 2012. The name and symbol had previously been proposed for element 102 but were not accepted by IUPAC at that time.
It is a transactinide in the p-block of the periodic table. It is in period 7 and is the heaviest known member of the carbon group. Initial chemical studies in 2007–2008 indicated that flerovium was unexpectedly volatile for a group 14 element. More recent results show that flerovium's reaction with gold is similar to that of copernicium, showing it is very volatile and may even be gaseous at standard temperature and pressure. Nonetheless, it also seems to show some metallic properties, consistent with it being the heavier homologue of lead.
Very little is known about flerovium, as it can only be produced one atom at a time, either through direct synthesis or through radioactive decay of even heavier elements, and all known isotopes are short-lived. Six isotopes of flerovium are known, ranging in mass number between 284 and 289; the most stable of these, Fl, has a half-life of ~2.1 seconds, but the unconfirmed Fl may have a longer half-life of 19 seconds, which would be one of the longest half-lives of any nuclide in these farthest reaches of the periodic table. Flerovium is predicted to be near the centre of the theorized island of stability, and it is expected that heavier flerovium isotopes, especially the possibly magic Fl, may have even longer half-lives.

Introduction

History

Pre-discovery

In the late 1940s to early 1960s, the early days of making heavier and heavier transuranic elements, it was predicted that since such elements did not occur naturally, they would have shorter and shorter spontaneous fission half-lives, until they stopped existing altogether around element 108. Initial work in synthesizing the heavier actinides seemed to confirm this. But the nuclear shell model, introduced in 1949 and extensively developed in the late 1960s by William Myers and Władysław Świątecki, stated that protons and neutrons form shells within a nucleus, analogous to electron shells. Noble gases are unreactive due to a full electron shell; similarly, it was theorized that elements with full nuclear shells – those having "magic" numbers of protons or neutrons – would be stabilized against decay. A doubly magic isotope, with magic numbers of both protons and neutrons, would be especially stabilized. Heiner Meldner calculated in 1965 that the next doubly magic isotope after Pb was Fl with 114 protons and 184 neutrons, which would be the centre of an "island of stability". This island of stability, supposedly from copernicium to oganesson, would come after a long "sea of instability" from mendelevium to roentgenium, and the flerovium isotopes in it were speculated in 1966 to have half-lives over 10⁸ years. These early predictions fascinated researchers, and led to the first attempt to make flerovium, in 1968 with the reaction. No flerovium atoms were detected; this was thought to be because the compound nucleus only has 174 neutrons instead of the supposed magic 184, and this would have significant impact on the reaction cross section and half-lives of nuclei produced. It was then 30 more years before flerovium was first made. Later work suggests the islands of stability around hassium and flerovium occur because these nuclei are respectively deformed and oblate, which make them resistant to spontaneous fission, and that the true island of stability for spherical nuclei occurs at around unbibium-306.
In the 1970s and 1980s, theoretical studies debated whether element 114 would be a more volatile metal like lead, or an inert gas.

First signs

The first sign of flerovium was found in December 1998 by a team of scientists at Joint Institute for Nuclear Research, Dubna, Russia, led by Yuri Oganessian, who bombarded a target of plutonium-244 with accelerated nuclei of calcium-48:
This reaction had been tried before, without success; for this 1998 attempt, JINR had upgraded all of its equipment to detect and separate the produced atoms better and bombard the target more intensely. One atom of flerovium, alpha decaying with lifetime 30.4 s, was detected. The decay energy measured was 9.71 MeV, giving an expected half-life of 2–23 s. This observation was assigned to and was published in January 1999. The experiment was later repeated, but an isotope with these decay properties was never observed again, so the exact identity of this activity is unknown. It may have been due to the isomer Fl, but because the presence of a whole series of longer-lived isomers in its decay chain would be rather doubtful, the most likely assignment of this chain is to the 2n channel leading to Fl and electron capture to Nh. This fits well with the systematics and trends of flerovium isotopes, and is consistent with the low beam energy chosen for that experiment, though further confirmation would be desirable via synthesis of Lv in a Cm reaction, which would alpha decay to Fl. The RIKEN team reported possible synthesis of Lv and Fl in 2016 in a Cm reaction, but the alpha decay of Lv was missed, alpha decay of Fl to Cn was observed instead of electron capture to Nh, and the assignment to Lv instead of Lv was not certain.
Glenn T. Seaborg, a scientist at Lawrence Berkeley National Laboratory who had been involved in work to make such superheavy elements, had said in December 1997 that "one of his longest-lasting and most cherished dreams was to see one of these magic elements"; he was told of the synthesis of flerovium by his colleague Albert Ghiorso soon after its publication in 1999. Ghiorso later recalled:
Seaborg died two months later, on 25 February 1999.
In March 1999, the same team replaced the Pu target with Pu to make other flerovium isotopes. Two atoms of flerovium were produced as a result, each alpha-decaying with a half-life of 5.5 s. They were assigned as Fl. This activity has not been seen again either, and it is unclear what nucleus was produced. It is possible that it was an isomer Fl or from electron capture by Fl, leading to Nh and Rg.

Confirmed discovery

The now-confirmed discovery of flerovium was made in June 1999 when the Dubna team repeated the first reaction from 1998. This time, two atoms of flerovium were produced; they alpha decayed with half-life 2.6 s, different from the 1998 result. This activity was initially assigned to ²⁸⁸Fl in error, due to the confusion regarding the previous observations that were assumed to come from ²⁸⁹Fl. Further work in December 2002 finally allowed a positive reassignment of the June 1999 atoms to ²⁸⁹Fl.
In May 2009, the Joint Working Party of IUPAC published a report on the discovery of copernicium in which they acknowledged discovery of the isotope ²⁸³Cn. This implied the discovery of flerovium, from the acknowledgement of the data for the synthesis of ²⁸⁷Fl and ²⁹¹Lv, which decay to ²⁸³Cn. The discovery of flerovium-286 and -287 was confirmed in January 2009 at Berkeley. This was followed by confirmation of flerovium-288 and -289 in July 2009 at Gesellschaft für Schwerionenforschung in Germany. In 2011, IUPAC evaluated the Dubna team's 1999–2007 experiments. They found the early data inconclusive, but accepted the results of 2004–2007 as flerovium, and the element was officially recognized as having been discovered.

Isotopes

While the method of chemical characterization of a daughter was successful for flerovium and livermorium, and the simpler structure of even–even nuclei made confirmation of oganesson straightforward, there have been difficulties in establishing the congruence of decay chains from isotopes with odd protons, odd neutrons, or both. To get around this problem with hot fusion, the decay chains from which terminate in spontaneous fission instead of connecting to known nuclei as cold fusion allows, experiments were done in Dubna in 2015 to produce lighter isotopes of flerovium by reaction of ⁴⁸Ca with ²³⁹Pu and ²⁴⁰Pu, particularly ²⁸³Fl, ²⁸⁴Fl, and ²⁸⁵Fl; the last had previously been characterized in the ²⁴²Pu²⁸⁵Fl reaction at Lawrence Berkeley National Laboratory in 2010. ²⁸⁵Fl was more clearly characterized, while the new isotope ²⁸⁴Fl was found to undergo immediate spontaneous fission, and ²⁸³Fl was not observed. This lightest isotope may yet conceivably be produced in the cold fusion reaction ²⁰⁸Pb²⁸³Fl, which the team at RIKEN in Japan at one point considered investigating: this reaction is expected to have a higher cross-section of 200 fb than the "world record" low of 30 fb for ²⁰⁹Bi²⁷⁸Nh, the reaction which RIKEN used for the official discovery of element 113. Alternatively, it might be produced in future as a great-granddaughter of ²⁹⁵120, reachable in the ²⁴⁹Cf reaction. The reaction ²³⁹Pu+⁴⁸Ca has also been suggested as a means to produce ²⁸²Fl and ²⁸³Fl in the 5n and 4n channels respectively, but so far only the 3n channel leading to ²⁸⁴Fl has been observed.
The Dubna team repeated their investigation of the ²⁴⁰Pu+⁴⁸Ca reaction in 2017, observing three new consistent decay chains of ²⁸⁵Fl, another decay chain from this nuclide that may pass through some isomeric states in its daughters, a chain that could be assigned to ²⁸⁷Fl, and some spontaneous fissions of which some could be from ²⁸⁴Fl, though other interpretations including side reactions involving evaporation of charged particles are also possible. The alpha decay of ²⁸⁴Fl to spontaneously fissioning ²⁸⁰Cn was finally observed by the Dubna team in 2024.