High Quality Content by WIKIPEDIA articles! Roentgenium (Rg) 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 272Rg in 1994, which is also the only directly synthesized isotope, all others are decay products of ununtrium, ununpentium, and ununseptium. There are 7 known radioisotopes from 272Rg to 282Rg. The longest-lived isotope is 281Rg with a half-life of 26 seconds.