High Quality Content by WIKIPEDIA articles! Actinium (Ac) has no stable isotopes, thus a standard atomic mass cannot be given. There are 31 known isotopes, from 206Ac to 236Ac, and 2 isomers. Two isotopes are found in nature, 227Ac and 228Ac, as intermediate decay products of, respectively, 235U and 232Th. 228Ac is extremely rare, and almost all natural actinium is 227Ac.