Fermium-255

Main isotopes of fermium

Iso­tope Decay abun­dance half-life mode energy (MeV) pro­duct 252Fm syn 25.39 h SF – – α 7.153 248Cf 253Fm syn 3 d ε 0.333 253Es α 7.197 249Cf 255Fm syn 20.07 h SF – – α 7.241 251Cf 257Fm syn 100.5 d α 6.864 253Cf SF – –

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 2 nuclear isomers, ^250mFm and ^251mFm. The longest-lived isotope is ²⁵⁷Fm with a half-life of 100.5 days, and the longest-lived isomer is ^250mFm with a half-life of 1.8 seconds.

²⁶⁰Fm? was not confirmed in 1997.

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