Einsteinium, a radioactive metallic chemical element. Einsteinium is man-made and does not occur naturally on earth. It is made chiefly by neutron bombardment of plutonium in a nuclear reactor, a slow process that produces only tiny amounts of einsteinium. Although it has no commercial uses, einsteinium serves as the source of mendelevium, another radioactive element. The half-life of einsteinium's most stable isotope, einsteinium 252, is only about 472 days.

Einsteinium was discovered in 1952 by a group of scientists that included Albert Ghiorso, Stanley G. Thompson, and Glenn T. Seaborg. The element was found in debris gathered from the explosion of the first thermonuclear, or hydrogen, bomb. It was named for Albert Einstein.

Symbol: Es. Atomic number: 99. Atomic weight of most stable isotope: about 252. Einsteinium has 14 known isotopes: Es-243 through Es-256. It is a transuranium element belonging to the actinide series of the Periodic Table.