Isotopes of fermium

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 (in fallout from nuclear testing) 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 (²⁶⁰Fm is unconfirmed), 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.