Fermium

Fermium is a radioactive actinide with the configuration 5f12 7s2. No piece of fermium metal has ever been made: the largest samples are picograms of fermium-257, its longest-lived isotope with a half-life of 100.5 days. Tracer experiments show that it forms Fm3+ in solution and, like its heavier neighbours, can be reduced to Fm2+. Its chemistry resembles erbium in the lanthanide row, and its position in elution sequences from ion-exchange columns helped confirm its identity.

Fermium marks a hard limit for reactor production. Heavier elements are built in reactors by adding neutrons one at a time and waiting for beta decay to raise the atomic number. At fermium this process stops: fermium-257 does not beta-decay, and the next isotope, fermium-258, undergoes spontaneous fission within about 370 microseconds. This 'fermium gap' means every element beyond fermium must be made in accelerators by fusing nuclei, a few atoms at a time.

Discovery

Fermium was found in coral debris from the Ivy Mike hydrogen bomb test of November 1952, a few weeks after einsteinium, by Albert Ghiorso's team with colleagues at Argonne and Los Alamos. They identified fermium-255, with a 20-hour half-life, formed when uranium absorbed 17 neutrons. The discovery stayed classified until 1955 and was named after Enrico Fermi, who had died in November 1954.