An artificially produced, highly unstable, transuranic rare earth element of the actinide series with no detectable natural occurrence on Earth.

Einsteinium was first identified in 1952 from the debris of the first thermonuclear explosion of a hydrogen bomb which took place on a Pacific atoll. The isotope einsteinium-253 was produced which decays by alpha decay and has a halflife of 20.5 days. In 1961 10 nanograms of einsteinium had been produced, and was used to produce mendelevium. In the 1970's 3 micrograms of einsteinium-253 were produced by irradiating kilograms of plutonium-239 with neutrons in a nuclear reactor to produce plutonium-242, which was subsequently made into plutonium oxide pellets and mixed with aluminium powder and further irradiated with neutrons for a year to produce californium-253, an alpha/beta decaying isotope with a halflife of 18 days, some of which decays by the beta decay route into einsteinium-253.

Einsteinium exhibits two valences, +2 and the commonly expressed +3. Among the compounds known are einsteinium sesquioxide, Es₂O₃, einsteinium trichloride, EsCl₃, a tribromide, EsBr₃, and einsteinium oxychloride, EsOCl.

The longest lived isotope of einsteinium is einsteinium-252 with a halflife of 1.29 years decaying by either alpha decay into the beta/inverse beta decaying berkelium-248 which has a halflife of 24 hours or by inverse beta decay into the alpha/spontaneous fission decaying californium-252 which has a halflife of 2.6 years.

Altogether, 14 isotopes of einsteinium are known, all radioactive, and ranging from the inverse beta/alpha decaying einsteinium-243 which has a halflife of 21 seconds to the beta decaying einsteinium-256 with a halflife of 25 minutes.

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