Marie Curie first isolated the element radium in pure metallic form. She and Pierre Curie announced radium in 1898, but initially obtained only tiny amounts mixed with other substances.
Their raw material was pitchblende residue, which remained unusually radioactive even after uranium had been removed. That clue showed that the ore contained powerful radioactive elements in minute quantities. Radium was especially difficult to separate because it behaves chemically much like barium.
After years of processing and repeated fractional crystallisation, Curie isolated radium metal in 1910 with André-Louis Debierne. This achievement helped earn her the 1911 Nobel Prize in Chemistry, awarded for the discovery of radium and polonium, the isolation of radium, and her study of its compounds.
A common mix-up is polonium: Curie discovered and named it in 1898, after Poland, but she never isolated it in pure form because its short half-life made the task exceptionally difficult.