Marie Curie isolated pure radium metal in 1910.
The year 1898 marks the Curies’ announcement of radium’s discovery, but discovering the element and isolating it as a pure metal were separate achievements. Marie Curie and Pierre Curie initially found radium in pitchblende residues after uranium had been removed. Because radium behaves chemically much like barium, separating the two elements required repeated fractional crystallisation and years of painstaking processing.
In September 1910, Marie Curie and André-Louis Debierne announced that they had isolated radium metal. Their method involved electrolysing a solution of pure radium chloride with a mercury cathode, forming a radium–mercury amalgam. Heating the amalgam in hydrogen removed the mercury and left metallic radium.
The date is sometimes confused with 1898, when the Curies announced radium’s existence, or 1911, when Curie received the Nobel Prize in Chemistry for her work on radium and polonium. The 1903 Nobel Prize recognised her earlier research on radiation, shared with Pierre Curie and Henri Becquerel. Thus, 1910 is the appropriate answer for isolating the pure metal.