High Quality Content by WIKIPEDIA articles! Rutherfordium (Rf) 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 either 259Rf in 1966 or 257Rf in 1969. There are 15 known radioisotopes from 253Rf to 268Rf (2 of which, 266Rf and 268Rf, are unconfirmed) and 4 isomers. The longest-lived isotope is 267Rf with an estimated half-life of 5 hours. The longest directly measured half-life is 263Rf at 11 minutes, and the longest-lived isomer is 261mRf with a half-life of 81 seconds.