Marie Curie, Nobel Physicist and Chemist
In November 1891, at nearly twenty-four years old, Marie boarded a train for Paris from her native Poland with a new first name, changing Maria to the French Marie. At the Sorbonne, she was in her element. She excelled in physics and mathematics, and went on to graduate at the top of her class, and was offered a scholarship to stay at the Sorbonne, and earn the equivalent of a BS in mathematics. She could not turn this down; science was her life.
In the winter of 1893, Marie wrestled with what would become a perennial problem — the search for more laboratory space. When some Polish friends visited in spring 1894, she moaned about the conditions. The friends knew of someone doing similar research nearby, a Frenchman named Pierre Curie.
When she got to know Pierre, Marie’s life changed forever. Pierre was different from any man she had known; intelligent and quiet, he loved science as much as she did. Pierre’s family, just like Marie’s, had placed a huge emphasis on education, but he hadn’t followed a conventional path—leaving the Sorbonne in 1893 to teach at a new industrially oriented school.
Pierre was equally bowled over by Marie, seeing in her a startling intelligence. Their friendship quickly deepened, and soon Pierre told Marie that he wanted to marry her. The following summer she decided to leave Paris for Warsaw, feeling that she wasn’t prepared to marry. She felt a sense of duty both to her family and to Poland. But Pierre was not prepared to give up on this remarkable woman. He wrote, begging her to return to Paris, and even offered to leave France and move to Poland. It is perhaps this offer, more than anything else, that made Marie understand Pierre’s feelings. She returned to commit herself to Pierre and a life in France. Pierre decided finally to write up his doctoral thesis on types of magnetism, and in March 1895 was awarded his PhD and a professorship. Later that spring, Marie made the final commitment. The pair married on July 26, 1895, at Sceaux town hall in Paris. From the beginning, the couple worked together as often as they could. French mathematician Henri Poincaré once said that their relationship was not just an exchange of ideas but also “an exchange of energy, a sure remedy for the temporary discouragements faced by every researcher.” In early 1897, Marie discovered that she was pregnant, and suffered frequent dizzy spells and sickness.
In a paper presented in 1898 Marie first theorized that the rays from uranium were an atomic property.
Shortly after they returned to Paris from a break in Brittany, Marie went into labor and, on September 12, 1897, gave birth to a daughter, Irène.
By now, Marie had qualified to be a teacher and was assembling charts and photographs for her article on the magnetism of tempered steel for the bulletin of the Society for the Encouragement of National Industry. She and Pierre decided that she should begin original research and prepare for a doctorate. The previous two years had been exciting in physics. In 1895, Wilhelm Röntgen discovered X-rays, leaving physicists scrambling to understand this strange new phenomenon. And, in 1896, in what should have been an X-ray experiment, Henri Becquerel uncovered what were first known as Becquerel rays. Marie decided that she would study these new rays for her doctorate, since they the were not well understood.
With Pierre’s help, Marie set up a laboratory in an old storeroom. It was cold and dirty, but Marie was happy to devote herself to a research topic of her choosing. She began her laboratory notebook on December 16, 1897. She and Pierre built an ionization chamber to measure the energy given off by the uranium. These measurements were extremely tricky — Becquerel himself had failed — but Marie believed that with care and diligence she would succeed, and she did.
She began her work with the intention of submitting a PhD dissertation in which she would measure quantities more accurately than before, with no expectations of new discoveries. After testing uranium and measuring the tiny electrical charges in the mysterious Becquerel rays, she scavenged other elements to test. On one day alone in February 1898 she tested thirteen elements, including gold and copper, finding that none gave off uranium rays. If Marie had stuck to testing pure elements, she would have missed the discovery that would make her famous, but on February 17, she tried a sample of the black heavy mineral compound pitchblende. This had been mined for over a century from the mineral-rich Joachimsthal region on the German-Czech border. In 1789, Martin Heinrich Klaproth had extracted a gray metallic element from pitchblende, which he named “uranium” after the newly discovered planet Uranus. As uranium was a relatively small component of pitchblende, Marie expected rays from the pitchblende to be weaker than those from pure uranium. To her amazement, she found the reverse. Initially she thought she had made a mistake, but the result was confirmed after checking. But why was the radiation from pitchblende stronger? Marie tested other substances and, a week later, made another unexpected discovery. The mineral aeschynite, which contains thorium but no uranium, was also more active than uranium. Now she had two puzzles to solve.
In 1898 the Curies discovered two new elements: polonium, named in honor of Marie’s home country, and radium.
Marie suspected that the rays that Becquerel had discovered were not just a phenomenon of uranium, but something more general. The obvious assumption was that there was another, more energetic, element in the pitchblende, also giving off these rays. But what could this element be? Pitchblende contained a mix of minerals, too many to duplicate in the laboratory. About this time, the Curies discovered that another uranium-containing mineral, chalcite, also gave off more energetic rays than pure uranium. Chalcite was simpler to synthesize than pitchblende and the Curies reasoned that, if they created chalcite from its known ingredients, the mystery component would be missing and the rays would be less energetic. Marie mixed up artificial chalcite by combining copper phosphate and uranium, and found that the new mineral showed no greater activity than uranium. The conclusionwas clear — chalcite and pitchblende contained an additional unknown component. Marie wrote up her findings as “Rays Emitted by Uranium and Thorium Compounds.” It was read to the French Academy on April 12, 1898. Neither Marie nor Pierre was a member of the Academy and couldn’t present, but Marie’s professor and good friend, Gabriel Lippmann, was willing to give the paper. The Academy members were intrigued by Marie’s findings but did not pick up on two points in the paper that, in retrospect, were the most important.
Marie had conjectured that there was a new component in the pitchblende and chalcite that was responsible for the increased energy of the emitted rays. This introduced a novel technique for detecting new elements—that the radioactive properties of a material could indicate their presence. We now know that there are ninety-two naturally occurring elements, but in 1898 far fewer were known. The presence of other, undiscovered elements was suggested by “gaps” in the periodic table that Dmitri Mendeleev had introduced to the world of chemistry in 1869, and as each decade passed since his pioneering work, more of the gaps had been filled.Secondly, in the paper Marie stated, “All uranium compounds are active... the more so, in general, the more uranium they contain.” Implicit in this statement is the suggestion that the rays were an atomic property — an idea that turned out to be prophetic. But the Academicians were not convinced of the existence of a new component. The only way to prove this would be to isolate the new component, which could be a new element.
A few days after her paper was read, Marie and Pierre were back in their laboratory, pulverizing 100 grams of pitchblende to try to isolate the mysterious new element(s). They treated the pitchblende with various chemicals, measuring the activity of the products of chemical reactions.
The Curies would admire the eerie glow of their samples of radium in their lab.
The most active of the breakdown products was then worked on further. Two weeks after starting, the Curies felt that they had isolated enough active product to determine its atomic weight using spectroscopy. This would show conclusively if they had found a new element.
By now, the laboratory notebook suggests that Marie and Pierre were dividing their efforts. Very soon, they had a product that was seventeen times more active than the uranium that they used as a benchmark. By June 25, Marie had a product that was 300 times more active than uranium. Pierre, working in parallel, isolated a product 330 times more active.
The Curies were now beginning to think that pitchblende contained not one but two new elements– one associated with bismuth in the ore, the other with barium. Feeling they had isolated enough of the element associated with bismuth, they tried spectroscopy again, calling upon spectroscopy expert Eugène-Anatole Demarçay, but no new signature was found. Despite this lack of evidence, the Curies were convinced that the bismuth harbored an unknown element. On July 13, 1898, Pierre wrote a significant entry in the notebook. It is the first indication that they had given their hypothetical element a name —“Po”—an abbreviation for “polonium,” the name the Curies chose in honor of Marie’s home country.
By the end of November, they had isolated a highly active product, carried off by barium. With Bémont’s help, they increased its radioactivity to 900 times that of uranium. This time Demarçay found what they hoped for: distinct spectral lines that could not be attributed to a known element. In late December, Pierre wrote the name for this second new element in the middle of the page of their notebook — radium.
Rather than continuing to work together on the same project, they set up in parallel. In early 1899, Marie worked on isolating radium while Pierre attempted to understand the nature of radioactivity better. He took on the physics, while Marie handled the chemistry. Marie had a stubborn desire to isolate a sample of radium, but she also knew that to win over the skeptics they had to isolate their new elements.
Marie had to resort to industrial-type methods, requiring a bigger laboratory. The Curies asked the Sorbonne, but all the university could offer was an abandoned building formerly used as a dissection laboratory. The huge space had no heating, so in the winter it was horribly cold. Pierre and Marie huddled around a small stove to keep warm, then hurried into the cold partsof the laboratory to conduct their work. There were no extractor hoods to carry away the poisonous gases given off by Marie’s chemical treatment, so work had to be done in the courtyard. If the weather didn’t allow it, they worked inside, opening the windows.
By spring 1899 Marie had the materials needed. As she later recounted: “I had to work with as much as twenty kilograms of material at a time... so the hangar was filled with great vessels full of precipitates and of liquids. It was exhausting work to move the containers about, to transfer the liquids and to stir for hours at a time, with an iron bar, the boiling material in the cast-iron basin.”
Despite the arduous work and long hours, Marie thrived on the challenge. As she worked on isolating the radium, the Curies had an unexpected source of delight — concentrated radium compounds were spontaneously luminous. Sometimes, after supper, the couple would wander back to their laboratory to admire the eerie glow of their samples.
As yet, no one realized the harm the substance could do, but Marie and Pierre were already feeling its ill effects. Pierre’s hands became so damaged from handling radium that he had difficulty dressing himself. His bones ached and he walked like a man twenty or thirty years older. Marie, too, was often weak. It seems strange that neither made any connection between their deteriorating health and the radiation with which they worked. In a paper Pierre wrote during this period, he noted that laboratory animals breathing the emanations of radioactive substances in a confined space died within a matterof hours. The paper concluded, “We have established the reality of a toxic action from radium emanations introduced into the respiratory system.”
Despite Marie’s frequent bouts of weakness and ill health, in December 1904, she gave birth to her second daughter, Ève. Marie herself would die from overexposure to radiation in 1934.
Pierre’s work was also progressing. He reported the effects of magnetic fields on radium emissions and the Curies published a stream of papers. At the 1900 International Congress of Physics in Paris, they presented their longest paper yet, “The New Radioactive Substances,” in which they summarized their findings, plus work from England and Germany.
By this time, it was known that some of the rays could be deflected by a magnet while others could not, and some rays penetrated thick barriers that others could not. And radioactive elements could “induce” radioactivity in other substances — turning the Curies’ laboratory radioactive. But no one knew how any of this worked. As the paper said, “The spontaneity of the radiation is an enigma, a subject of profound astonishment.”
Isolating radium from pitchblende was exhausting and time-consuming.
During this long undertaking, Marie presented two progress reports in scientific journals and then, finally, in 1902, announced that she had successfully isolated one-tenth of a gram of radium chloride. Her paper announced that the measured atomic weight of radium was 225, close to the current agreed value of 226, and concluded that “according to its atomic weight, [radium] should be placed in the Mendeleev [periodic] table after barium in the column of the alkaline earth metals.”
Marie’s isolation of radium was not only a huge achievement for sheer doggedness; it was crucial in developing our understanding of radioactivity. Physicist Jean Perrin noted in 1924,“It is not an exaggeration to say today that [the isolation of radium] is the cornerstone on which the entire edifice of radioactivity rests.”
Marie has gone down in history as one of the true greats of science, man or woman. As a female scientist she was a pioneer: the first woman to win a Nobel Prize; the first to become a professor at the Sorbonne; the first to direct a major science research institute; and the first to be buried in the Pantheon. She was also a pioneer in combining motherhood with a full-time career in science, paving the way for countless women who came after her.
Adapted from 10 Women Who Changed Science and the World by Catherine Whitlock and Rhodri Evans (Diversion Books, 2019).
