fermium

/ferr"mee euhm/, n. Chem., Physics.
a transuranic element. Symbol: Fm; at. no.: 100.
[1950-55; named after E. FERMI; see -IUM]

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 (Fm), synthetic chemical element of the actinoid series of the periodic table, atomic number 100. Fermium (as the isotope fermium-255) is produced by the intense neutron irradiation of uranium-238 and was first positively identified by Albert Ghiorso and coworkers at Berkeley, Calif., in debris taken from the first thermonuclear or hydrogen-bomb test explosion (November 1952), in the South Pacific. All fermium isotopes are radioactive. Mixtures of the isotopes fermium-254 (3.24-hour half-life), fermium-255 (20.1-hour half-life), fermium-256 (2.7-hour half-life), and fermium-257 (80-day half-life) can be produced by the intensive slow-neutron irradiation of elements of lower atomic number, such as plutonium.

      The stability of the isotope fermium-257 makes it possible to work with weighable amounts of fermium. Fermium-250 (30-minute half-life), the alpha decay product of nobelium, served to ascertain the existence of nobelium-254. Fermium exists predominantly in the +3 oxidation state; there is also some evidence for the +2 state.

atomic number
100
stablest isotope
257
oxidation states
+2, +3
electronic config.
[Rn]5f127s2

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Universalium. 2010.

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