Isotopes of fermium

Nuclides with atomic number of 100 but with different mass numbers

Fermium (100Fm) 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 255Fm in 1952. 250Fm was independently synthesized shortly after the discovery of 255Fm. There are 20 known radioisotopes ranging in atomic mass from 241Fm to 260Fm, and 4 nuclear isomers, 247mFm, 250mFm, 251mFm, and 253mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 247mFm with a half-life of 5.1 seconds.

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