Roentgenium was discovered by research scientists at the Heavy Ion Research Laboratory in Darmstadt, Germany in 1994.
Roentgenium is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first synthesized isotope was 272Rg in 1994, which is also the only directly synthesized isotope. There are 7 known radioisotopes from 272Rg to 282Rg. The longest-lived isotope is 281Rg with a half-life of 26 seconds.