Hassium was discovered by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research at Darmstadt, Germany in 1984.
Hassium is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was 265Hs in 1984. There are 12 known isotopes from 263Hs to 277Hs and 1-4 isomers. The longest-lived isotope is 269Hs with a half-life of 9.6 seconds.