mendelevium

Synthesized, radioactive metallic element of the actinide series, atomic number 101, relative atomic mass 258. It was first produced by bombardment of Es-253 with helium nuclei. Its longest-lived isotope, Md-258, has a half-life of about two months. The element is chemically similar to thulium. It was named by the US physicists at the University of California at Berkeley who first synthesized it in 1955 after the Russian chemist Dmitri Mendeleyev, who in 1869 devised the basis for the periodic table of the elements.