Ununpentium was discovered in February 2004 by Russian scientists at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany and American scientists at the Lawrence Livermore National Laboratory.
Ununpentium 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 288Uup in 2004. There are five known radioisotopes from 287Uup to 291Uup.