einsteinium (ĪnˈstĪˌnēəm, ĪnstĪˈ–) [key] [for Albert Einstein], 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 table; its chemical properties are believed to be similar to those of the other members of the actinide series. The seventh transuranium element 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-life 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 mendelevium. 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.