High Quality Content by WIKIPEDIA articles! Ununpentium (Uup) 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 four known radioisotopes from 287Uup to 290Uup. The longest-lived isotope is 289Uup with a half-life of 220 ms.