High Quality Content by WIKIPEDIA articles! Rubidium (Rb) has 32 isotopes, with naturally occurring rubidium being composed of just two isotopes; 85Rb (72.2%) and the radioactive 87Rb (27.8%). Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 to 60 days. Standard atomic mass is 85.4678(3) u.