High Quality Content by WIKIPEDIA articles! Seaborgium (Sg) 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 263mSg in 1974. There are 12 known radioisotopes from 258Sg to 271Sg and 2 known isomers (261mSg and 263mSg). The longest-lived isotope is 271Sg with a half-life of 2.4 minutes.