To definitively confirm that radium was well and truly a new element, the Curies needed to measure its atomic weight.1 In the early 20th century, atomic weight represented an element’s relative weight compared to that of another reference element. This weight is what makes each element unique.
To determine the atomic weight of radium, which involved a series of extremely precise weighings, the Curies first had to extract the purest radium possible from uranium ore. They therefore needed to get their hands on some uranium ore.
Over several years, they managed to obtain residues from uranium mines in Joachimsthal (now Jáchymov), Bohemia, free of charge. The Curies received two tons of residues in 1899, six tons in 1900 and five tons in 1902. By 1902, after a great deal of effort, they had managed to extract 0.1 gram of radium chloride from the 13 tons of residues.
Marie Curie conducted a series of experiments to measure radium’s atomic weight. In 1903, she published a dissertation in which she wrote: From these experiments, I have determined that radium’s atomic weight is Ra = 225. I consider this number to be accurate within one unit. 2 This number is consistent with the currently accepted value of 226.0 atomic mass units. The fact that no other element had the same atomic weight confirmed the discovery of radium. In 1924, Marie Curie wrote: The radium-bearing barium was extracted in the factory, and I carried on its purification and fractional crystallization in the laboratory. In 1902 I succeeded in preparing a decigram of chloride of pure radium which gave only the spectrum of the new element, radium. I made a first determination of the atomic weight of this new element, an atomic weight much higher than that of barium. Thus, the chemical individuality of radium was completely established, and the reality of radio elements was a known fact about which there could be no further controversy. 3
1Now referred to as atomic mass. Atomic mass represents the average mass of an element’s natural, stable isotopes, weighted using their relative abundance.
2From Marie Curie’s dissertation, Recherches sur les substances radioactives (Research on Radioactive Substances), 1903.
3Marie Curie, Pierre Curie, 1924.