The periodic table, developed initially by Mendeleev in 1869, organizes elements into a remarkably valuable framework to classify and organize many types of chemical behavior. Generally speaking, elements in vertical columns have similar chemical properties. The table has found wide application in physics, biology, engineering, and industry.
At present, there are 117 known elements. Of these, 80 have at least one stable isotope, these being the first 82 elements, except for technetium and promethium. Lead, with an atomic number of 82, is the heaviest stable element. The other 37 elements are radioactive, but some decay sufficiently slowly that they can be found in significant quantities in nature. In order of scarcity, the naturally occurring radioactive elements are thorium, uranium, bismuth, actinium, protactinium, radium, polonium, radon, technetium, neptunium, plutonium, promethium, astatine, and francium. Their abundance in the Earth’s crust ranges from thorium, present at about 1 ppm, to francium, which is so scarce that the entire Earth’s crust contains less than a pound.
We have now accounted for the first 94 elements of the periodic table. These are all elements which either were, or could have been, discovered before the era of nuclear physics. What remain are the 23 artificially synthesized elements. These come in two groups – the actinide elements having atomic numbers from 95 to 103 and the transactinide elements having atomic numbers from 104 to 118, save for element 117, which has not yet been synthesized. In 2008, an international team led by Amnon Marinov of the Hebrew University of Jerusalem claimed to have found atoms of element 122 in naturally occurring thorium, but this claim has been met with near-universal skepticism.
While the actinide elements were first synthesized either by intense neutron bombardment of lower atomic number elements (at least 2 were first discovered in nuclear fallout) or by bombardment with light ions, the transactinides were all first synthesized by bombardment with high energy heavy ions ranging in atomic number from neon to zinc. This probably makes it fair to call the 14 known transactinides the ‘new’ elements on the periodic table, as they required a new approach to their synthesis and also represent a complete new chemical series.
Three primary teams are involved in this research, and credit for the discovery of a new element is often shared between them. In general, making fewer than 50 atoms form the basis for claiming discovery, and can be as small as 3 or 4 atoms. Because of the similar discovery paths and the extremely limited information available on the transactinide elements, the tale will be told in tabular form.
Atomic # Name Date Team # isotopes Half-lives
104 Rutherfordium 1973 LBNL/JINR 16 13 hours – 50 microsec
105 Dubnium 1977 LBNL/JINR 16 32 hours – 0.5 second
106 Seaborgium 1993 LBNL 16 1 hour – 3 millisec
107 Bohrium 1989 GSI 16 90 min – 300 microsec
108 Hassium 1984 GSI 15 1 hour – 540 microsec
109 Meitnerium 1982 GSI 15 30 min – 1 millisec
110 Darmstadtium 1994 GSI 15 11 sec – 3 microsec
111 Roentgenium 1994 GSI 12 10 min – 2 millisec
112 Copernicium 2005 GSI 9 40 min – 1 millisec
113 Ununtrium 2004 LLBL/JINR 7 20 min – 70 millisec
114 Ununquadium 1999 JINR/LLNL 5 5 sec – 160 microsec
115 Ununpentium 2004 JINR/LLNL 5 1 min – 32 millisec
116 Ununhexium 2005 FLNR 5 60 millisec–6 millisec
118 Ununoctium 2005 LBNL 2 900 microsec
LBNL – Lawrence Berkeley National Laboratory
LLNL – Lawrence Livermore National Laboratory
JINR – Joint Institute for Nuclear Research (Dubna, Russia)
GSI – Gesellschaft für Schwerionenforschung (Darmstadt, Germany)
The date of discovery is when the experiment which confirmed synthesis was performed, not when the IUPAC/IUPAP Joint Working Party (JWP) made an official announcement.
The chemical properties of the transactinides may not follow the trends predicted by the periodic table because of relativistic effects – the valence electrons move sufficiently rapidly to alter their energy levels. Some relativistic effects are seen in the regular elements, including the low chemical reactivity of gold and that mercury is a liquid.
Considering the small amount of the transactinides elements synthesized and the very short radioactive half-lives, it is amazing that the chemical properties of several of these super-heavy elements have been studied. Both gas phase and liquid chemical analysis have been carried out. Elements 104, 105, 106, 107, 108, 112, and 114 have all had at least basic chemical reactivity studies performed, often using only a few atoms. Studies of element 114, ununquadium, carried out with only 3 atoms of 114, have shown chemical properties similar to those of an inert gas, instead of the expected Group IV chemical properties.
Finally, there is a prediction that ‘islands of stability’ exist among elements of large atomic number. Some researchers believe there are signs of a small increase in stability among the heaviest few transactinides, but this is still a subject of controversy. There may be isotopes having rather long half-lives among the elements with atomic numbers above 120. If so, there is the possibility of entering an entirely new subfield of chemistry. Despite the difficulty of the task, there is great promise for exciting discovery in the study of super-heavy elements.
