(redirected from Element 99)
Also found in: Dictionary, Thesaurus, Medical.


(īn`stī'nēəm, īnstī`–) [for Albert EinsteinEinstein, Albert
, 1879–1955, American theoretical physicist, known for the formulation of the relativity theory, b. Ulm, Germany. He is recognized as one of the greatest physicists of all time.
..... Click the link for more information.
], artificially produced radioactive chemical element; symbol Es; at. no. 99; mass no. of most stable isotope 252; m.p. about 860°C;; b.p. and sp. gr. unknown; valence +2, +3. Einsteinium is a member of Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information.
; its chemical properties are believed to be similar to those of the other members of the actinide seriesactinide series,
a series of radioactive metallic elements in Group 3 of the periodic table. Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
..... Click the link for more information.
. The seventh transuranium elementtransuranium elements,
in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ.
..... Click the link for more information.
 to be discovered, einsteinium was isolated in Dec., 1952, by Albert Ghiorso and his coworkers at the Univ. of California at Berkeley in residue from the first thermonuclear test explosion in the South Pacific. They identified einsteinium-253, which has a half-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
..... Click the link for more information.
 of 20.5 days. It was not until 1961 that a weighable quantity (about 0.01 microgram) of the element was separated; it was used to prepare the element mendeleviummendelevium
, artificially produced radioactive chemical element; symbol Md; at. no. 101; mass no. of most stable isotope 258; m.p. 827°C;; b.p. and sp. gr. unknown; valence +1, +2, +3. Mendelevium is a metal of the actinide series in Group 3 of the periodic table.
..... Click the link for more information.
. Weighable quantities of einsteinium have since been prepared by neutron bombardment of plutonium. Seventeen isotopes, all of which are radioactive, are known. Einsteinium-252, the most stable isotope, has a half-life of 472 days.



Es, a synthetic radioactive chemical element, a member of the actinide series; atomic number, 99. Einsteinium has no stable isotopes; it has known isotopes with mass numbers 243 through 256.

The seventh transuranium element to be discovered, einsteinium was identified by A. Ghiorso and others in December 1952 in the debris from a thermonuclear explosion in the course of work involving members of the University of California Radiation Laboratory, the Argonne National Laboratory, and the Los Alamos Scientific Laboratory (USA). The isotope produced was 253Es, with a half-life of 20.5 days, originating from the beta decay of 253U and daughters; the 235U was formed by the essentially successive capture of 15 neutrons by 238U.

Einsteinium can be investigated with macroscopic quantities using the isotopes 253Es (half-life 20.5 days), 354Es (half-life 276 days), and 255Es (half-life 38.3 days), whose production by the irradiation of lighter elements is severely limited because of the required long sequence of neutron capture reactions over long periods of time in high-neutron-flux reactors. Most of the investigations have used the short-lived 253Es because of its greater availability, but the use of 254Es will increase as it becomes more available. In any case, the investigation of einsteinium is very difficult because of the high specific radioactivity and small available quantities of the isotopes.

Einsteinium metal, which is quite volatile, can be prepared by the reduction of EsF3 with lithium and has a face-centered cubic crystal structure. The melting point is 860±30°C.

Einsteinium exists in normal aqueous solution in most stable form as Es +3 (green), although Es +2 can be produced under strong reducing conditions. The Es+3/Es+2 reduction potential is estimated to be − 1.24 ± 0.2 volt on the scale in which the hydrogen ion-hydrogen potential is 0.0 volt.

Numerous solid compounds, such as Es2O3, EsCl3, EsOCl, EsBr2, EsBr3, Esl2, and Esl3, have been synthesized and studied.

The electron structure of the gaseous element is 5f117s2 (beyond the radon structure).


Seaborg, G. T. lskusstvennye transuranovye elementy. Moscow, 1965. (Translated from English.)



Synthetic radioactive element, symbol Es, atomic number 99; discovered in debris of 1952 hydrogen bomb explosion; now made in cyclotrons.


a metallic transuranic element artificially produced from plutonium. Symbol: Es; atomic no.: 99; half-life of most stable isotope, 252Es: 276 days