The transuranium (or transuranic) elements are the chemical elements with atomic number greater than 92, which is the atomic number of uranium. All of them are radioactively unstable and decay into other elements. They are synthetic and none occur naturally on Earth, except for neptunium and plutonium which have been found in trace amounts in naturally occurring uranium ore.
Overview
Of the elements with atomic numbers 1 to 92, most can be found in nature, having stable isotopes (such as lead) or very long-lived radioisotopes (such as uranium), or existing as common decay products of the decay of uranium and thorium (such as radium). The exceptions are technetium, promethium, astatine, and francium; all four occur in nature, but only in very minor branches of the uranium and thorium decay chains, and thus all except francium were first discovered by synthesis in the laboratory rather than in nature. All elements with higher atomic numbers have been first discovered in the laboratory, with neptunium and plutonium (the first two of these) later discovered in nature. They are all radioactive, with a half-life much shorter than the age of the Earth, so any primordial (i.e. present at the Earth's formation) atoms of these elements have long since decayed. Trace amounts of neptunium and plutonium form in some uranium-rich rock, and small amounts are produced during atmospheric tests of nuclear weapons. These two elements are generated by neutron capture in uranium ore with subsequent beta decays (e.g. 238U + n → 239U → 239Np → 239Pu). All elements beyond plutonium are entirely synthetic, at least on Earth; they are created in nuclear reactors or particle accelerators. The half-lives of these elements show a general trend of decreasing as atomic numbers increase. There are exceptions, however, including several isotopes of curium and dubnium. Some heavier elements in this series, around atomic numbers 110–114, are thought to break the trend and demonstrate increased nuclear stability, comprising the theoretical island of stability. Transuranic elements are difficult and expensive to produce, and their prices increase rapidly with atomic number. As of 2008, the cost of weapons-grade plutonium was around $4,000/gram, and californium exceeded $60,000,000/gram. Einsteinium is the heaviest element that has been produced in macroscopic quantities. Transuranic elements that have not been discovered, or have been discovered but are not yet officially named, use IUPAC's systematic element names. The naming of transuranic elements may be a source of controversy.
Discoveries
So far, essentially all transuranium elements have been discovered at four laboratories: Lawrence Berkeley National Laboratory (LBNL) in the United States (elements 93–101, 106, and joint credit for 103–105), the GSI Helmholtz Centre for Heavy Ion Research in Germany (elements 107–112), RIKEN in Japan (element 113), and the Joint Institute for Nuclear Research (JINR) in Russia (elements 102 and 114–118, and joint credit for 103–105).
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