Marie Curie: radium, polonium and two Nobel Prizes

In 1898 two new elements were added to the periodic table, not because anyone had seen them, but because an instrument had measured more radiation than the known elements could explain. The person who trusted that measurement was Marie Curie.

This article tells her story: her education in Warsaw and Paris, the method she used to find polonium and radium, the four years it took to isolate radium, her two Nobel Prizes, her work in the First World War and the price she paid for working with radiation.

From Warsaw to Paris

Maria Skłodowska grew up in Warsaw, then part of the Russian Empire, where universities did not admit women. She studied in the clandestine Flying University, a network of secret classes, and worked as a governess to help pay for her sister Bronisława’s medical studies in Paris.

In 1891 she moved to Paris and enrolled at the Sorbonne. She finished her degree in physics in 1893, first in her class, and a degree in mathematics in 1894. That year she met Pierre Curie, a physicist who studied crystals and magnetism. They married in 1895.

A thesis on mysterious rays

In 1896 Henri Becquerel found that uranium salts give off rays that fog photographic plates, even in the dark. Marie chose these “uranium rays” as the subject of her doctoral thesis.

Instead of using photographic plates, she measured how much the rays made the surrounding air conduct electricity. For that she used a sensitive electrometer with a piezoelectric quartz, invented by Pierre and his brother Jacques. It gave numbers, not just a darkened plate.

Her measurements led to a key idea: the strength of the rays depended only on the amount of uranium, not on whether it was wet or dry, solid or dissolved, or combined with other elements. The radiation came from the atoms themselves. She called the phenomenon radioactivity. Read more about it in What is Radioactivity.

Polonium and radium

Testing many minerals, she found that pitchblende, a uranium ore, was more radioactive than its uranium content could explain. Something else in it had to be radioactive — something no one had identified.

Pierre left his own research to join her. They dissolved the ore and separated it chemically into fractions, measuring the radioactivity of each one. The activity followed two different elements:

• July 1898 — Polonium. Found with the bismuth fraction and named after Poland, which at the time was not an independent country.
• December 1898 — Radium. Found with the barium fraction, together with Gustave Bémont, and named from the Latin radius, “ray”.

This was a new way to discover elements: follow the radiation, not a colour or a weight. A spectroscopist, Eugène Demarçay, confirmed a new spectral line for radium.

Four years in a shed

Chemists wanted to see radium itself and measure its atomic weight. The problem was quantity: radium was present in pitchblende at well under one part in a million.

The Curies obtained tonnes of pitchblende residues from a mine in Joachimsthal (now Jáchymov, Czech Republic) and worked in a draughty former dissecting shed at the School of Physics and Chemistry in Paris. Marie handled the heavy chemistry, stirring boiling masses with an iron rod. Radium and barium are chemically alike, so she separated them by fractional crystallization: radium chloride is slightly less soluble than barium chloride, so each crystallization enriched it a little more.

In 1902 she had about 0.1 g of radium chloride and measured radium’s atomic weight as 225 (today’s value for its most stable isotope is 226). The glowing samples on their shelves, she later wrote, looked like “faint, fairy lights”.

Two Nobel Prizes

In 1903 Marie defended her doctoral thesis and, the same year, shared the Nobel Prize in Physics with Pierre Curie and Henri Becquerel for their work on radioactivity. She was the first woman to receive a Nobel Prize.

In April 1906 Pierre was killed in a street accident in Paris. Marie took over his position and became the first woman to teach at the Sorbonne. In 1910, with André-Louis Debierne, she isolated radium as a pure metal.

In 1911 she received the Nobel Prize in Chemistry, alone, for the discovery of polonium and radium and the isolation of radium. She is still the only person to have won Nobel Prizes in two different sciences.

The “little Curies” of the First World War

When war broke out in 1914, Curie saw that X-rays could help surgeons find bullets and shrapnel. She organized mobile X-ray units in cars, which soldiers called petites Curies, learned to drive, trained around 150 women as X-ray operators and worked at the front with her daughter Irène. Their units are estimated to have examined over a million wounded soldiers.

The cost of radiation

Nobody in 1898 knew how harmful radiation was. The Curies carried radium samples in their pockets and kept them in desk drawers. Marie suffered burns, cataracts and long illnesses. She died on 4 July 1934 of aplastic anaemia, very likely caused by years of exposure.

Her laboratory notebooks are still radioactive today, contaminated mainly with radium-226. They are kept in lead-lined boxes at the National Library of France, and readers must sign a waiver and wear protective clothing.

Legacy

• The curie (Ci), a unit of radioactivity equal to 3.7 × 10¹⁰ decays per second, honours the Curies.
• Element 96, curium, was named after Marie and Pierre in 1944.
• Her daughter Irène Joliot-Curie won the 1935 Nobel Prize in Chemistry with Frédéric Joliot for creating artificial radioactivity.
• In 1995 her remains were moved to the Panthéon in Paris, making her the first woman honoured there for her own achievements.