einsteinium

Synthesized, radioactive, metallic element of the actinide series, atomic number 99, relative atomic mass 254.09.

It was produced by the first thermonuclear explosion, in 1952, and discovered in fallout debris in the form of the isotope Es-253 (half-life 20 days). Its longest-lived isotope, Es-254, with a half-life of 276 days, allowed the element to be studied at length. It is now synthesized by bombarding lower-numbered transuranic elements in particle accelerators. It was first identified by US chemist Albert Ghiorso and his team who named it in 1955 after Albert Einstein, in honour of his theoretical studies of mass and energy.