fermium fûrˈmēəm [key] [for Enrico Fermi], artificially produced radioactive chemical element; symbol Fm; at. no. 100; mass no. of most stable isotope 257; m.p. 1,527℃; b.p. and sp. gr. unknown; valence +2, +3. Fermium is a member of Group 3 of the periodic table. The physical properties of fermium are largely unknown; its chemical properties are believed to be similar to those of the other members of the actinide series. The eighth transuranium element to be discovered, fermium was first identified (1952) as fermium-255 (half-life about 20 hours) by Albert Ghiorso and his coworkers, who discovered it in residue from the first thermonuclear test explosion in the South Pacific. Twenty isotopes, all of which are radioactive, are known; the most stable is fermium-257, with a half-life of 100.5 days. Isotopes of fermium have been produced by neutron bombardment of plutonium.

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