transuranium elements, in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ. of California at Berkeley or at Argonne National Laboratory; in order of increasing atomic number they are neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium. Of these only neptunium and plutonium occur in nature; they are produced in minute amounts in the radioactive decay of uranium.

Much of the study of the transuranium elements has taken place at the Lawrence Berkeley National Laboratory (at Berkeley, Calif.) and at the Joint Institute for Nuclear Research in Dubna, Russia; workers at both locations share credit for the independent discovery of rutherfordium, dubnium, and seaborgium (at. no. 104, 105, and 106, respectively), which are the first three transactinide elements. A German team at the Institute for Heavy Ion Research at Darmstadt discovered bohrium, hassium, meitnerium, darmstadtium, roentgenium, and ununbium (at. no. 107 through 112). The Dubna laboratory, with assistance from Berkeley, claims to have synthesized ununquadium (at. no. 114), and working jointly with the Lawrence Livermore National Laboratory (at Livermore, Calif.) claims to have produced ununtrium (at. no. 113) and ununpentium (at. no. 115). The Berkeley team claimed to have produced ununhexium (at. no. 116) and ununoctium (at. no. 118), but later retracted the claim for ununoctium after other laboratories failed to reproduce Berkeley's results and a reanalysis of their data did not show the production of the element. Other research teams have since synthesized ununhexium directly.

Up to and including fermium (at. no. 100), the transuranium elements are produced by the capture of neutrons; the transfermium elements are synthesized by the bombardment of transuranium targets with light particles or, more recently, by projecting medium-weight elements at targets of other medium-weight elements (see also synthetic elements).

Isotopes of the transuranium elements are radioactive because their large nuclei are unstable, and the transactinide, or superheavy, elements in particular have very short half-lives. However, on the basis of theories of nuclear structure, physicists have predicted that certain transactinide elements may have relatively stable isotopes. For example, an isotope of element 114 with mass number 298 (comprising 114 protons and 184 neutrons) should be very stable and resemble lead in its chemical properties. However, the three isotopes of element 114 that are claimed to have been synthesized have fewer than the requisite 184 neutrons.

Any of the chemical elements after uranium in the periodic table (with atomic numbers greater than 92). All are radioactive (see radioactivity), with half-lives ranging from tens of millions of years to fractions of a millisecond. Only two, neptunium (93) and plutonium (94), occur in nature, and only as traces in uranium ores as a result of neutron irradiation. Transuranium elements with atomic numbers through 116, along with 118, have been produced in laboratories. Each appears to resemble the elements above it in the periodic table; in particular, the actinides, thorium (90) through lawrencium (103), are similar to the lanthanides, cerium (58) through lutetium (71). The naming of the transuranium elements has been fraught with controversy regarding which laboratory first made the discovery and should propose the name and whether elements should be named for living persons. Seealso Glenn Seaborg.

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