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(fûr`mēəm) [for Enrico FermiFermi, Enrico
, 1901–54, American physicist, b. Italy. He studied at Pisa, Göttingen, and Leiden, and taught physics at the universities of Florence and Rome. He contributed to the early theory of beta decay and the neutrino and to quantum statistics.
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], artificially produced radioactive chemical element; symbol Fm; at. no. 100; mass no. of most stable isotope 257; m.p. 1,527°C;; b.p. and sp. gr. unknown; valence +2, +3. Fermium is a member of Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. 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 seriesactinide series,
a series of radioactive metallic elements in Group 3 of the periodic table. Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
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. The eighth transuranium elementtransuranium elements,
in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ.
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 to be discovered, fermium was first identified (1952) as fermium-255 (half-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 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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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


Seaborg, G. T. Iskusstvennye transuranovye elementy. Moscow, 1965. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A synthetic radioactive element, symbol Fm, with atomic number 100; discovered in debris of the 1952 hydrogen bomb explosion, and now made in nuclear reactors.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a transuranic element artificially produced by neutron bombardment of plutonium. Symbol: Fm; atomic no.: 100; half-life of most stable isotope, 257Fm: 80 days (approx.)
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005