When Maria Skłodowska was born in Warsaw in 1867, Poland had not been an independent country for most of a century. Warsaw was in the part of Poland controlled by the Russian czar. Her parents were teachers. Women were not permitted to study at the University of Warsaw, so Maria and her sister joined a secret floating university, whose night classes met in changing places to avoid the czar's police. Russian authorities had forbidden Poles to teach laboratory science. So she learned chemistry from a chemist at the beet-sugar factory and, on Sundays and evenings, at a secret Museum that was really an illegal lab. Later she moved to Paris, France, to study at a university, and at the Sorbonne in Paris she began using the French name Marie. Her first name had been Maria.

In Paris, she met Pierre Curie in 1894 and married him in 1895. The two worked together as scientists. They found two new elements, polonium and radium. In 1898, they announced polonium in July and radium in December. Other scientists did not trust the announcement, because the Curies did not have enough of either to see and weigh. The existence of the elements was known only from their radioactivity. 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, and confirmation of both the technique of measurement and of the discoveries was in sight. A 1903 Nobel speech noted that it takes 1,000 kg of raw material to produce a few decigrams of the active substances polonium, radium and actinium.

Proof meant weighing. In 1902, Marie Curie isolated a decigram of pure radium. After great difficulties, she 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. By 1911, the Swedish Academy reported that she had determined radium's atomic weight to be 226.45. The result had a deeper meaning. In 1903, the Royal Swedish Academy of Sciences said the work of Becquerel and the Curies had given science knowledge of a property of matter quite new to it, the capability of emitting spontaneously these rays. In 1911, the Academy said radium's emanation breaks down into other products including helium. That established for the first time in the history of chemistry that one element can really be transmuted into another.

Industry made the work possible and spread it. In 1904, the French industrialist Armet de Lisle, whose factory would provide radium to doctors, began collaborating with the Curies. In 1911, Marie Curie noted that radium in pure salt form had come to be manufactured industrially, which she said had not been achieved for any other new radioactive substance. The Curies published how they isolated radium and did not patent their methods. 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. 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. Marie Curie died on 4 July 1934, of aplastic anemia, a blood disease that often results from exposure to large amounts of radiation. Her name lives on in a unit. The curie was originally defined as the quantity of radon in radioactive equilibrium with one gram of radium, and it was redefined in 1953 as the quantity of radioactive nuclide in which the number of disintegrations per second is 3.7 x 10^10. In 1920, Curie and colleagues created the Curie Foundation. Over the next two decades, it 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.

Words to know

• spectroscopist: a light analyst
• transmuted: changed into another
• ampule: a sealed vial
• decigram: tenth of gram
• nuclide: atom type