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The Collaborative International Dictionary of English v.0.48 (gcide)

Es \Es\ n.
the chemical symbol for {einsteinium}, a transuranic element
with atomic number 99. The atomic weight of the longest-lived
isotope, with a half-life of 276 days, is 254. The first
isotope discovered, having atomic weight 253 and a half-life
of 20 days, was recognized in 1952 in the debris from a
hydrogen bomb test. As much as 3 micrograms of einsteinium
were produced by a complex process involving long irradiation
of plutonium isotopes in nuclear reactors. Its chemical
properties are those of a trivalent actinide element. --HCP61

Syn: einsteinium, atomic number 99.
[WordNet 1.5 PJC]