High Quality Content by WIKIPEDIA articles! Ununtrium (Uut) 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 284Uut as a decay product of 288Uup in 2003. The first isotope to be directly synthesized was 278Uut in 2004. There are 6 known radioisotopes from 278Uut to 286Uut. The longest-lived isotope is 286Uut with a half-life of 19.6 seconds.