Isotopes of fermium


is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered was Fm in 1952. Fm was independently synthesized shortly after the discovery of Fm.
There are 20 known radioisotopes ranging in atomic mass from Fm to Fm, and 5 nuclear isomers. The longest-lived isotope is Fm with a half-life of 100.5 days, and the longest-lived isomer is Fm with a half-life of 5.1 seconds.

List of isotopes

Chronology of isotope discovery

IsotopeDiscoveredReaction
241Fm2008204Pb
242Fm1975204Pb, 206Pb
243Fm1981206Pb
244Fm1967233U
245Fm1967233U
246Fm1966235U
247Fm1967239Pu
248Fm1958240Pu
249Fm1960238U
250Fm1954238U
251Fm1957249Cf
252Fm1956249Cf
253Fm1957252Cf
254Fm1954Neutron capture
255Fm1954Neutron capture
256Fm1955Neutron capture
257Fm1964Neutron capture
258Fm1971257Fm
259Fm1980257Fm
260Fm?1992?254Es+18O, 22Ne — transfer

260Fm was not confirmed in 1997.