Atomic Number: 99
Atomic Symbol: Es
Atomic Weight: (252)
Electron Configuration: -29-8-2
Einsteinium, the seventh transuranic element of the actinide series to be discovered, ws identified by Ghiorso and co-workers at Berkeley in December 1952 in debris from the first large thermonuclear explosion, which took place in the Pacific in November, 1952.
The isotope produced was the 20-day 253Es isotope. In 1961, a sufficient amount of einsteinium was produced to permit separation of a macroscopic amount of 253Es.
This sample weighted about 0.01Mg. A special magnetic-type balance was used in making this determination.
253Es so produced was used to produce mendelevium (Element 101). About 3Mg of einsteinium has been produced at Oak Ridge National Laboratories by irradiating for several years kilogram quantities of 239Pu in a reactor to produce 242Pu.
This was then fabricated into pellets of plutonium oxide and aluminum powder, and loaded into target rods for an initial 1-year irradiation at the Savannah River Plant, followed by irradiation in a HFIR (High Flux Isotopic Reactor).
After 4 months in the HFIR the targets were removed for chemical separation of the einsteinium from californium.
Fourteen isotopes of einsteinium are now recognized.
254Es has the longest half-life (275 days).
Tracer studies using 253Es show that einsteinium has chemical properties typical of a heavy trivalent, actinide element.