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 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.  [WordNet 1.5]