The atom was believed to be the most elementary particle, a word meaning undivided or indivisible. The discovery of the electron by other scientists hinted that this ancient idea was false, but nobody yet grasped the inner structure or the immense energy stored in atoms. Marie Curie was born Maria Skłodowska in Warsaw, and she moved to Paris, France, to study at a university. In 1896, Henri Becquerel found that uranium gives off rays. She chose uranium rays for her thesis because so little had been published on them: with no long list of papers to read, she could start experiments right away.
To measure the faint electric currents that uranium rays create in air, Marie used an electrometer invented about 15 years earlier by Pierre Curie and his older brother Jacques. She found that the strength of the rays depended only on how much uranium was present. She went beyond Becquerel by forming a crucial hypothesis: that the rays from uranium compounds could be an atomic property of uranium, built into the very structure of its atoms. In April 1898, she found that thorium compounds, like those of uranium, give off rays, and she invented the word radioactivity, based on the Latin word for ray, to describe this behavior. She also found that radiation does not depend on how atoms are arranged in molecules, so the ability to emit radiation must be linked to the interior of the atom itself.
A pitchblende puzzle sent her toward new elements. She measured that certain pitchblendes containing 75% uranium oxide were about four times as radioactive as uranium oxide, which suggested that a small quantity of a highly radioactive unknown material was present. The Curies' method was to separate pitchblende into chemical parts and measure the radioactivity of each part, following each chemical separation with a measurement of the activity of the products. In July 1898, they reported that the bismuth part held a new element, which acted almost exactly like bismuth chemically but was radioactive, so it had to be something new. Other scientists did not trust the announcement of polonium and radium because the Curies did not have enough of either to see and weigh. The existence of the elements was known only from their radioactivity.
Proof therefore meant isolation. Marie explained that the Curies' conviction that polonium and radium were new elements rested on the atomic nature of radioactivity, and that to prove it they had to isolate the hypothetical elements, which for radium took several years of unremitting effort. The spectroscopist Eugène Demarçay at first found no new spectral lines in the Curies' material, so more ore was needed; later he observed a new line in the spectrum. A 1903 Nobel speech noted that it takes 1,000 kg of raw material to produce a few decigrams of the active substances. In 1902, Marie isolated a decigram of pure radium and determined its atomic weight for the first time as 225, close to the value of 226 that the Dictionary of Scientific Biography gives as the recognized one. She never succeeded in isolating polonium; for reasons not understood until radioactive decay was developed, polonium has a half-life of only 138 days.
The deeper claim was harder to accept. At first, the Curies were not convinced that radioactive energy came from inside atoms; they considered, for example, that Earth might be bathed in cosmic rays whose energy certain atoms caught and radiated. The Curies observed that every atom of a radioactive body acts as a constant source of energy, which implied a revision of the principles of conservation, and the press exclaimed that with radium the Curies had discovered perpetual motion. Marie was not ready to accept Rutherford and Soddy's idea that radioactive substances destroy themselves at an exponential rate until experiments showed radium did not act on its surroundings to produce atomic transformations; the idea implied radium's apparent stability meant a very long period of decay. Her 1910 treatise on radioactivity then admitted, without reservations, the theory of transformations, and she introduced the terms disintegration and transmutation in her teaching.
In 1906, Lord Kelvin suggested that radium was not an element but a compound of lead and five helium atoms. Because this threatened the science of radioactivity, Marie began lab work to disprove it. With the help of her colleague André Debierne, she confirmed that radium was an element, though it took years to measure its atomic weight beyond question. By 1911, the Swedish Academy reported that she had determined radium's atomic weight to be 226.45, and it said radium's emanation breaks down into other products including helium, which established for the first time in the history of chemistry that one element can really be transmuted into another. In her 1911 Nobel lecture, Marie said the most direct proof that radioactive elements undergo atomic transformations was the formation of the chemically defined element helium from the chemically defined element radium. She said that the history of radium had proved her hypothesis that radioactivity is an atomic property of matter and can provide a means of seeking new elements, and that it let scientists predict about 30 new elements.
Credit and prizes were part of the story. French academics first proposed only Pierre Curie and Henri Becquerel for the 1903 physics prize, but Pierre insisted that his wife share the honor, and Marie became the first woman to win a Nobel Prize. The 1903 Nobel citation was worded to avoid mentioning polonium and radium, because chemists on the committee thought the discovery might later deserve a Nobel Prize in Chemistry and doubts remained about elements isolated only in invisibly small amounts. In 1911, she won that second prize, in chemistry, and the Royal Swedish Academy of Sciences noted that it was the first time in the eleven years of the Nobel Prizes that the distinction had been conferred upon a previous prizewinner. In her lecture, she said that radium and polonium had been found by Pierre Curie in collaboration with her, and that the chemical work of isolating radium was carried out specially by her but was intimately connected with their common work. As of November 2026, she is still the only individual to have received a Nobel Prize in two different science categories.
Her research policies were to push the precision and rigor of measurement to the extreme, to obtain samples that are pure or of maximum concentration even if that meant handling enormous quantities of raw materials, and to state general laws only when there were no exceptions. Industry helped meet that need. In 1904, the French industrialist Armet de Lisle, whose factory would provide radium to doctors, began collaborating with the Curies, who gave technical suggestions on treating pitchblende and in return could accumulate larger samples of radioactive material. André Debierne adapted their lab techniques into larger industrial processes, and a chemical company took a share of the radium salts extracted in exchange for supplying chemicals and paying staff. In 1910, Marie was charged with preparing an ampule of about twenty milligrams of radium metal for the International Bureau of Weights and Measures in Paris, to serve as the standard for radium. The Curies published how they isolated radium and did not patent their methods.
Danger was understood slowly. In his 1905 Nobel address, Pierre Curie warned that radium, like Nobel's explosives, might prove very dangerous in the wrong hands, but said he believed mankind would derive more good than harm from the new discoveries. In the 1920s, products such as the Tho-Radia beauty cream contained radioactive thorium and radium, and through the 1920s most people believed that low levels of radiation were beneficial. The Curies did not fully understand how dangerous the radioactive materials they handled were. In 1925, Marie joined a French Academy of Medicine commission that recommended lead screens and periodic blood-cell tests for workers in industrial labs where radioactive materials were prepared. Marie Curie died on 4 July 1934, of aplastic anemia, a blood disease that often results from exposure to large amounts of radiation. Her chemistry laboratory at the Curie Museum in Paris was reconstructed in 1981 and decontaminated, because it was too radioactive for safe occupancy.
The organization she built outlasted her. She organized the Radium Institute as an entire major laboratory devoted to a single subject, with small research teams that each asked their own questions about radioactivity and trained students. Between 1919 and 1934, scientists there published 483 works, including 31 papers and books by Curie herself, and her staff always included some foreigners, especially Poles, and some women. In 1920, Curie and colleagues created the Curie Foundation, which over the next two decades became a major international force in the treatment of cancer. As of November 2026, the Curie Institute in Paris is a center for cancer treatment and research, and the U.S. Department of Energy says radioactivity is used as a source of heat for satellites, for medical imaging, for targeted cancer treatments, and for radiometric dating. A notebook of Marie Curie's from 1899 to 1902 is still radioactive as of November 2026 and, the Nobel Prize site says, will be for 1,500 years.
Words to know
- hypothesis: a testable idea
- pitchblende: a uranium ore
- transmutation: one element changing
- electrometer: a current gauge
- radioactivity: rays from atoms
