roentgenium, artificially produced radioactive chemical element; symbol Rg; at. no. 111; mass number of most stable isotope 280; m.p., b.p., sp. gr., and valence unknown. Situated in Group 11 of the periodic table, it is expected to have properties similar to those of gold.

In 1994 an international research team led by Peter Armbruster and Sigurd Hofmann at the Institute for Heavy Ion Research at Darmstadt, Germany bombarded bismuth-209 atoms with nickel-64 ions. In an 18-day experiment, three atoms were unambiguously identified as an isotope of element 111 with mass number 272 and a half-life of 1.5 msec. The discovery was officially confirmed in 2003, and the discoverers named the element in honor of Wilhelm Conrad Roentgen. The most stable isotope, roentgenium-280, has a half-life of approximately 3.6 sec.

See also synthetic elements; transactinide elements; transuranium elements.

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