Definition: einsteinium

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Source: WordNet (r) 1.7 

einsteinium
     n : a radioactive transuranic element produced by bombarding
         plutonium with neutrons [syn: Es, E, atomic number
         99]

Source: Elements database 20001107 

einsteinium
Symbol: Es
Atomic number: 99
Atomic weight: (254)
Appearance is unknown, however it is most probably metallic and silver
or gray in color. Radioactive metallic transuranic element belonging to the
actinoids. Es-254 has the longest half-life of the eleven known isotopes at
270 days. First identified by Albert Ghiorso and associates in the debris of
the 1952 hydrogen bomb explosion. In 1961 the first microgram quantities of
Es-232 were separated. While einsteinium never exists naturally, if a
sufficient amount was assembled, it would pose a radiation hazard.