Fermium

Fermium is a synthetic chemical element; it has symbol Fm and atomic number 100. It is an actinide and the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not been prepared yet. A total of 20 isotopes are known, with ²⁵⁷Fm being the longest-lived with a half-life of 100.5 days.

Fermium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Enrico Fermi, one of the pioneers of nuclear physics. Its chemistry is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

Discovery

Fermium was first discovered in the fallout from the 'Ivy Mike' nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Initial examination of the debris from the explosion had shown the production of a new isotope of plutonium, ²⁴⁴Pu: this could only have formed by the absorption of six neutrons by a uranium-238 nucleus followed by two β⁻ decays. At the time, the absorption of neutrons by a heavy nucleus was thought to be a rare process, but the identification of ²⁴⁴Pu raised the possibility that still more neutrons could have been absorbed by the uranium nuclei, leading to new elements.