Roentgenium was first observed in 1994 and several isotopes have been synthesized since its first discovery. The most stable known isotope is 281Rg with a half-life of ~20 seconds, which decays by spontaneous fission, like many other N=170 isotones.
Roentgenium was officially discovered by an international team led by Sigurd Hofmann at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany on December 8, 1994. Only three atoms of it were observed (all 272Rg), by the cold fusion between nickel ions and a bismuth target in a linear accelerator:
20983Bi + 6428Ni --> 272111Rg + 10n
In 2001, the IUPAC/IUPAP Joint Working Party (JWP) concluded that there was insufficient evidence for the discovery at that time. The GSI team repeated their experiment in 2002 and detected a further 3 atoms. In their 2003 report, the JWP decided that the GSI team should be acknowledged as the discoverers.
The name roentgenium (Rg) was proposed by the GSI team in honor of the German physicist Wilhelm Conrad Röntgen, and was accepted as a permanent name on November 1, 2004. Previously the element was known under the temporary IUPAC systematic element name unununium, Uuu.
The heavier members of this group are well known for their lack of reactivity or noble character. Silver and gold are both inert to oxygen, but are attacked by the halogens. In addition, silver is attacked by sulfur and hydrogen sulfide, highlighting its higher reactivity compared to gold. Roentgenium is expected to be even more noble than gold and can be expected to be inert to oxygen and halogens. The most-likely reaction is with fluorine to form a trifluoride, RgF3.
Check out this interactive Periodic Table.
Check out this Roentgenium video. Prepared by The University of Nottingham.