Fm, a synthetic radioactive chemical element, the eleventh member of the actinide series. Atomic number, 100. Fermium has no stable isotopes; isotopes with mass numbers from 244 to 258 are known.
Fermium, named after E Fermi, was first identified by A. Ghiorso and his co-workers in January 1953 in the form of the isotope 255Fm (half-life, 20.1 hours), which was in the debris from a thermonuclear explosion. The identification involved scientists of the Radiation (Lawrence Berkeley) Laboratory of the University of California, the Los Alamos Scientific Laboratory, and the Argonne National Laboratory. The isotope originated from the beta decay of 255U, which had formed from the successive capture of 17 neutrons by 238U.
No isotope of fermium has yet been isolated in weighable amounts, and thus all the investigations of the element have been done with tracer quantities. The longest-lived isotope is 257Fm (half-life, approximately 80 days), whose production in high-neutron-flux reactors is extremely limited because of the very long sequence of neutron-capture reactions required. In addition, the extremely unavailable isotope 254Fm (half-life, 3.24 hours) has been identified in the “metallic” zero-valent state in an atomic-beam magnetic resonance experiment, an identification that established the electron structure of elemental fermium in the ground state as 5f27s2. It is surmised that this is the structure of the gaseous element as well. In normal aqueous solution, fermium, as is the case with such other heavy actinides as einsteinium and mendelevium, displays an oxidation state of +3; however, strong reducing conditions can produce Fm2+, which has greater stability than Es2+ but less stability than Md2+. The Fm3+/Fm2+ reduction potential is estimated to be –1.1 ±0.2 volts on the scale in which the hydrogen ion/hydrogen potential is 0.0 volts.
G. T. SEABORG (USA)