DENSITY: unknown

Fermium, the eleventh member of the actinide series, was discovered in 1952. Fermium (element 100), together with einsteinium (element 99), were unexpectedly produced in the explosion of the first U.S. thermonuclear device, "Mike," tested at Eniwetok Atoll in the South Pacific on November 1, 1952. Fermium was first identified in the form of the 255 Fm (half-life 20 hours). The name fermium was chosen for element 100, in honor of the great physicist Enrico Fermi. Fermium isotopes of masses 242 through 260 are known. All are radioactive, decaying via α -particle emission, electron capture, and spontaneous fission . The isotope of mass 258 has the shortest half-life (0.4 milliseconds), and that of mass 257 has the longest (100.5 days). It is noteworthy that a predominantly symmetric mass division (during spontaneous fission) was first observed in 259 Fm. The ground state electronic configuration of the gaseous fermium atom is [Rn]5f 12 7s 2 , analogous to that of its lanthanide homologue (erbium). The most stable ion in aqueous solution is Fm 3+ , although Fm 2+ and Fm 4+ have been reported. However, the claim of the identification of the latter two ions has not been substantiated. As with einsteinium, divalency is seen in the metal .

SEE ALSO Actinium ; Berkelium ; Einsteinium ; Fermi, Enrico ; Lawrencium ; Mendelevium ; Neptunium ; Nobelium ; Plutonium ; Protactinium ; Radioactivity ; Rutherfordium ; Thorium ; Transactinides ; Transmutation ; Uranium .

Darleane C. Hoffman


Ghiorso, Albert; Thompson, S. G.; Higgins, G. H.; et al. (1955). "New Elements Einsteinium and Fermium, Atomic Numbers 99 and 100." Physical Review 99:1048[L].

Hoffman, Darleane C.; Ghiorso, Albert; and Seaborg, Glenn T. (2000). The Transuranium People: The Inside Story. Singapore: World Scientific Publishing.

Seaborg, Glenn T., and Loveland, Walter D. (1990). The Elements beyond Uranium. New York: Wiley.

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