Who first isolated radium chloride?

Marie Curie was the first person to isolate radium chloride as a distinct radium compound. Working with uranium ore called pitchblende, she separated minute quantities of radium from chemically similar barium through repeated crystallisation.

Radium was extraordinarily difficult to obtain because it occurred only in trace amounts and behaved much like barium. After years of processing large quantities of pitchblende residues, Curie and her collaborators produced a small amount of radium chloride. In 1902, about one-tenth of a gram had been separated from roughly a tonne of ore.

This achievement was an important step toward proving that radium was a new element. Marie Curie later isolated radium metal in 1910, using electrolysis of a radium chloride solution with a mercury cathode. That later accomplishment is sometimes confused with the earlier isolation of radium chloride itself.

André-Louis Debierne worked with Curie on the isolation of radium metal, but he is not the canonical answer to this question. Pierre Curie was Marie’s scientific partner and husband, yet the isolation work is conventionally credited to Marie Curie.