Einsteinium
. Einsteinium, symbol Es, artificially created radioactive element with an atomic number of 99. Einsteinium is one of the transuranium elements in the actinide series of the Periodic table . Isotopes of einsteinium with mass numbers ranging from 243 to 256 are known. The element, named in honor of German-born American physicist Albert Einstein , was discovered in 1952 in the debris produced by a thermonuclear explosion (see Nuclear Weapons ). The isotope first identified had an atomic mass of 253 and a half-life of 20 days. Subsequently, the most long-lived of all the known einsteinium isotopes, einsteinium-252, was prepared by irradiating plutonium in a nuclear reactor; however, only small amounts are now being produced (see Radioactivity ).