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Einsteinium is a synthetic element in the periodic table that has the symbol Es and atomic number 99. A metallic highly radioactive transuranic element (7th in the series) in the actinides, einsteinium is produced by bombarding plutonium with neutrons and was discovered in the debris of the first hydrogen bomb test. It was named after Albert Einstein and has no known uses. Tracer studies using the isotope Es-253 show that einsteinium has chemical properties typical of a heavy trivalent, actinide element. Einsteinium has 99 protons, 99 electrons and 153 neutrons.


Einsteinium was first identified in December 1952 by Albert Ghiorso at the University of California, Berkeley and another team headed by G.R. Choppin at Los Alamos National Laboratory. Both were examining debris from the first hydrogen bomb test of November, 1952 (see Operation Ivy). They discovered the isotope einsteinium-253 (half-life 20.5 days) that was made by the nuclear fusion of 15 neutrons with uranium-238 (which then went through seven beta decays). These findings were kept secret until 1955 due to Cold War tensions, however.

In 1961, enough einsteinium was synthesized to prepare a macroscopic amount of Es-253. This sample weighed about 0.01 mg and was measured using a special balance. The material produced was used to produce mendelevium. Further einsteinium has been produced at the Oak Ridge National Laboratory's High Flux Isotope Reactor in Tennessee by bombarding plutonium-239 with neutrons. Around 3 mg was created over a four year program of irradiation and then chemical separation from a starting 1 kg of plutonium isotope.


20 radioisotopes of einsteinium have been characterized, with the most stable being Es-252 with a half-life of 471.7 days, Es-254 with a half-life of 275.7 days, Es-255 with a half-life of 39.8 days, and Es-253 with a half-life of 20.47 days. All of the remaining radioactive isotopes have half-lifes that are less than 40 hours, and the majority of these have half lifes that are less than 30 minutes. This element also has 3 meta states, with the most stable being Es-254m (t½ 39.3 hours). The isotopes of einsteinium range in atomic weight from 241.0686620 amu (Es-241) to 257.0959790 amu (Es-257).


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