In 1896 Henri Becquerel found that uranium salts fogged a photographic plate in the dark. Marie Curie took up the phenomenon for her doctoral work and made the crucial measurement: using an electrometer built by her husband Pierre and his brother, she showed that the activity of a uranium compound was proportional to the amount of uranium in it and independent of its chemical form or of the conditions it was kept under. Whatever this was, it was a property of the atom and not of the molecule. She coined the word radioactivity.
She then found that pitchblende, the uranium ore, was substantially more active than its uranium content could account for. The only explanation was an unknown and more active substance present in small quantity. In July 1898 the Curies announced polonium, and on 26 December 1898 radium.
Announcing is not isolating. The Curies obtained tonnes of pitchblende residue from the mines at Jachymov in Bohemia and processed it by hand in an unheated shed, through repeated crystallisations, over four years. By 1902 they had a tenth of a gram of radium chloride and an atomic weight for it. Marie Curie received the Nobel Prize in Physics in 1903 with Pierre and Becquerel, and the Nobel Prize in Chemistry in 1911 for radium and polonium, and remains the only person to have won in two different sciences.
The Curies did not patent the isolation process, taking the view that scientific results should be freely available. Pierre was killed by a horse-drawn wagon in 1906. Marie died in 1934 of aplastic anaemia, consistent with prolonged radiation exposure; her notebooks are still too contaminated to handle without protection and are kept in lead-lined boxes.