Marie and Pierre Curie Discover Radium and Polonium (1898)

The discovery of radium and polonium by Marie and Pierre Curie in 1898 showed that radioactivity was a property of atoms and revealed elements far more intensely radioactive than uranium. Working in Paris, the Curies announced polonium in July and radium in December 1898, after chemically separating tonnes of uranium ore. The work earned Marie Curie two Nobel Prizes and transformed physics, chemistry and medicine, at a heavy cost to her own health.

The story of the discoveries is also a story of method. Marie Curie’s precise measurements turned radioactivity from a curiosity into a quantity that could be measured, and the couple’s chemical approach, following an invisible property through a long chain of separations, became the model for radiochemistry.

Background: Becquerel’s rays

In early 1896 Henri Becquerel found that uranium salts gave off rays that fogged photographic plates in the dark (see Becquerel’s discovery of radioactivity). Coming just after the discovery of X-rays, the effect attracted little lasting attention, and by 1897 few people were studying it.

Maria Skłodowska was born in Warsaw, then in the Russian-ruled part of Poland, on 7 November 1867. Women could not attend university in Russian Poland, so after years as a governess she moved to Paris in 1891 to study at the Sorbonne, earning degrees in physics (1893) and mathematics (1894). In 1894 she met Pierre Curie, a physicist known for work on crystals and magnetism, and they married on 26 July 1895. Their first daughter, Irène, was born in September 1897.

Measuring the rays

Late in 1897 Marie Curie chose Becquerel’s uranium rays as the subject of her doctoral thesis. Instead of photographic plates, she measured how strongly a sample made air conduct electricity, using an electrometer and a quartz device based on piezoelectricity, the effect discovered by Pierre and his brother Jacques in 1880. This gave precise numbers instead of rough images.

She soon established that the strength of the radiation depended only on the amount of uranium present, not on its chemical form or physical state. She concluded that the radiation came from the atoms themselves, a fundamental insight. She also found that thorium gave off similar rays; the German chemist Gerhard Carl Schmidt reported the same independently in 1898.

A clue in pitchblende

The crucial clue came from minerals. Pitchblende, a black uranium ore, and the mineral chalcolite were several times more active than their uranium content could explain. Marie Curie proposed, in a note presented to the Academy of Sciences on 12 April 1898, that they must contain an unknown and far more active element.

Pierre Curie set aside his own research to join her. They ground pitchblende, dissolved it in acids and separated it by standard chemical methods into groups of elements, measuring the activity of each fraction. The radioactivity concentrated in two places: one fraction that behaved chemically like bismuth, and another that behaved like barium.

Polonium and radium

In a paper presented on 18 July 1898, the Curies announced a new element in the bismuth fraction. They named it polonium after Poland, which at the time did not exist as an independent state. The same paper used the word “radio-active” for the first time.

On 26 December 1898, with the chemist Gustave Bémont, they announced a second element in the barium fraction, about 900 times more active than uranium in their sample. They called it radium, from the Latin radius, “ray”. The spectroscopist Eugène Demarçay found a new line in its spectrum, an important confirmation.

The amounts involved were tiny. Pitchblende contains only a fraction of a gram of radium per tonne of ore, and the Curies’ first radium samples were mostly barium with a trace of the new element. What made radium detectable at all was its extraordinary activity, which an electrometer could measure even when the element itself was invisible to ordinary chemical tests.

Chemists were cautious: no one had seen either element in pure form or measured its atomic weight. Proving radium’s existence became Marie Curie’s task for the next four years.

Four years in a shed

The work took place in a converted shed with a leaking glass roof at the School of Industrial Physics and Chemistry (ESPCI) in rue Lhomond, where Pierre taught. The German chemist Wilhelm Ostwald, who visited, later described it as something between a stable and a potato cellar.

With help from the Austrian government, the Curies obtained several tonnes of pitchblende residue from the mines of St Joachimsthal (now Jáchymov in the Czech Republic), left over after uranium had been extracted for colouring glass. Marie Curie processed batches of up to 20 kilograms at a time, stirring boiling mixtures with an iron rod, then performing thousands of crystallisations to separate radium from barium.

Radium preparations glowed faintly in the dark, and Marie Curie later wrote of the pleasure of entering the shed at night and seeing the luminous tubes on the shelves. The couple also had to support the family: Pierre taught at the school, Marie taught at a teacher-training college for women in Sèvres from 1900, and they chose not to patent their method of extracting radium, publishing it freely instead.

In 1902 she obtained about one tenth of a gram of pure radium chloride and measured radium’s atomic weight as 225 (the modern value for its most common isotope is 226). Polonium, with a half-life of only about 138 days, proved too short-lived to isolate in her lifetime by these methods.

The main people involved in the discoveries were:

• Marie Curie (1867–1934) designed the research programme, did most of the chemical separation and wrote the defining interpretation of radioactivity.
• Pierre Curie (1859–1906) contributed instruments and physical measurements; in 1903 he showed with Albert Laborde that radium continuously gives off heat.
• Gustave Bémont co-authored the radium announcement.
• André-Louis Debierne discovered actinium in 1899 and helped Marie Curie isolate radium metal in 1910.

Nobel Prizes and tragedy

Marie Curie defended her doctoral thesis on 25 June 1903. In December 1903 the Nobel Prize in Physics was shared between Becquerel and the Curies, making Marie the first woman to win a Nobel Prize. The committee had first planned to omit her; Pierre insisted she be included after a committee member alerted him.

On 19 April 1906 Pierre Curie was killed when he slipped and fell under a horse-drawn wagon in rue Dauphine, Paris. Marie took over his post and in November 1906 became the first woman to teach at the Sorbonne. In 1910, with Debierne, she isolated metallic radium.

In 1911 she received the Nobel Prize in Chemistry on her own, for discovering radium and polonium and for isolating radium. She remains the only person to have won Nobel Prizes in two different sciences. Earlier in 1911 she had been narrowly rejected for membership of the Academy of Sciences, and in the autumn parts of the French press attacked her over a relationship with the physicist Paul Langevin.

War work and later life

The Radium Institute in Paris was completed in 1914. During the First World War Marie Curie organised mobile X-ray units, nicknamed “petites Curies”, to help surgeons locate bullets and shrapnel. Around 20 vehicles and some 200 fixed radiology posts were set up, and she and her daughter Irène trained women as operators.

In 1921 she toured the United States, where President Warren Harding presented her with a gram of radium bought by public subscription. Irène Joliot-Curie and her husband Frédéric later won the 1935 Nobel Prize in Chemistry for creating artificial radioactivity.

The dangers of radiation

The Curies handled radium with no protection. Both suffered burns on their hands, and Pierre noted the injuries radium caused to skin, an observation that led to its medical use against tumours, known in France as curiethérapie. They also suffered chronic fatigue and illness that they did not link to their work.

Radium became a commercial craze in the early 20th century, added to tonics, cosmetics and luminous paint. Young women in American factories who painted watch dials with radium paint, and who shaped brushes with their lips, developed severe bone damage and cancers from the 1920s; their cases helped establish workers’ rights to compensation and stricter safety rules.

Marie Curie died on 4 July 1934 at the Sancellemoz sanatorium in Passy, Haute-Savoie, of aplastic anaemia, generally attributed to long exposure to radiation. In 1995 her remains and Pierre’s were moved to the Panthéon in Paris; she was the first woman honoured there for her own achievements.

Timeline

• 1867, 7 November: Maria Skłodowska is born in Warsaw.
• 1895, 26 July: Marie and Pierre Curie marry.
• 1898, 12 April: Marie Curie proposes that pitchblende contains a new element.
• 1898, 18 July: The Curies announce polonium.
• 1898, 26 December: The Curies and Bémont announce radium.
• 1902: Marie Curie isolates radium chloride and measures radium’s atomic weight.
• 1903, December: The Curies and Becquerel share the Nobel Prize in Physics.
• 1906, 19 April: Pierre Curie dies in a street accident in Paris.
• 1911, December: Marie Curie receives the Nobel Prize in Chemistry.
• 1934, 4 July: Marie Curie dies in Passy, Haute-Savoie.

Why it matters

Radium and polonium proved that atoms could release enormous energy and change into other elements, ideas that led to the nuclear model of the atom (see Rutherford’s discovery of the atomic nucleus) and later to nuclear fission. Marie Curie’s career also became a lasting symbol of women’s achievement in science, and the hazards she faced shaped modern radiation protection.