Marie Curie’s Discoveries: Radioactivity, Polonium, Radium and Who Found What

Ask most people who discovered radioactivity and they will say Marie Curie. The real story is better, and it has more people in it. Henri Becquerel found the rays. Marie Curie showed what they meant, named them, and with Pierre Curie found two new elements. Ernest Rutherford and Frederick Soddy then worked out that atoms were turning into other atoms. This post connects to our post on credit in physics, Who Really Discovered It?, and our Marie Curie sticker is the character who carries it.

What did Henri Becquerel discover in 1896?

Becquerel was studying how uranium salts are affected by light, prompted by the newly discovered X-rays. “By accident, he discovered that uranium salts spontaneously emit a penetrating radiation that can be registered on a photographic plate,” and further work showed it “was something new and not X-ray radiation” (NobelPrize.org^[1]). That was the discovery of the phenomenon. But nobody yet knew where the rays came from.

What did Marie Curie discover?

Radioactivity is an atomic property

Marie Curie’s choice of thesis topic was shaped by two recent discoveries: Wilhelm Roentgen’s X-rays and Becquerel’s uranium rays (AIP^[2]). Her tool was an electrometer that Pierre and his brother Jacques had invented about 15 years earlier, “a device for measuring extremely low electrical currents.” She used it to measure the faint currents that flow through air bombarded by uranium rays, in a damp storeroom where the moist air kept leaking the charge away (AIP^[2]).

Her measurements showed that the intensity depended on how much uranium a sample held. From this she formed what the American Institute of Physics calls a crucial hypothesis: that the emission “could be an atomic property of the element uranium—something built into the very structure of its atoms” (AIP^[2]). The AIP calls this simple hypothesis revolutionary.

She named it

In April 1898 she found that thorium compounds emit the same kind of rays. “To describe the behavior of uranium and thorium she invented the word ‘radioactivity’,” from the Latin word for ray (AIP^[2]).

Polonium and radium

The mineral pitchblende was more radioactive than its uranium content could explain, so the Curies concluded it held other radioactive substances (NobelPrize.org^[3]). Splitting it chemically and tracking the radioactivity with the electrometer, they found two hot fractions. In July 1898 they announced a new element in the bismuth fraction, named polonium in honour of the country of Marie’s birth, Poland. In December 1898 a second paper announced radium, in the barium fraction (AIP^[4]).

Proving it took industrial-scale work. They had to treat very large quantities of pitchblende, “a ton of which the Curies received as a donation from the Austrian government” (AIP^[4]). In 1910 Marie “successfully produced radium as a pure metal, which proved the new element’s existence beyond a doubt” (NobelPrize.org^[5]).

The work had beauty, too. The AIP exhibit quotes this memory of those nights: “One of our joys was to go into our workroom at night.... The glowing tubes looked like faint, fairy lights.” The Curies published all their methods for isolating radium “without patenting any of them” (AIP^[6]).

Who got the Nobel Prizes, and how were they shared?

The 1903 Nobel Prize in Physics was divided: one half to Henri Becquerel “in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity,” and the other half jointly to Pierre and Marie Curie “in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel” (NobelPrize.org^[7]). So Becquerel received 1/2, and each Curie 1/4.

The 1911 Nobel Prize in Chemistry went to Marie Curie alone (share 1/1) “in recognition of her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element” (NobelPrize.org^[8]). Marie, widowed in 1906, became “the first person ever to be awarded two Nobel Prizes” (NobelPrize.org^[5]). During World War I she organised mobile X-ray teams.

What did Rutherford and Soddy add?

The Curies found what was radioactive. Rutherford and Soddy explained what radioactivity is. In 1899 Rutherford showed “that there were at least two distinct types of radiation: alpha radiation and beta radiation.” Working with Soddy, “in 1902 they formulated a revolutionary theory: that elements could disintegrate and be transformed into other elements” (NobelPrize.org^[9]). Rutherford won the 1908 Nobel Prize in Chemistry “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.” Our Rutherford sticker remembers him.

Soddy went on to notice that some “new” radioactive substances were really variants of known elements, with the same chemistry but different radioactivity. “In 1913 Soddy introduced a term for this. He said that a substance can have different isotopes.” He was awarded the 1921 Nobel Prize in Chemistry and “received his Nobel Prize one year later, in 1922” (NobelPrize.org^[10]).

How fast does radium decay? The half-life law

Transformation theory gives a simple law. Each nucleus has a fixed chance per unit time, λ (the decay constant), of decaying. Starting from N₀ nuclei, the number left after time t is

N(t) = N₀e^(−λt)

The half-life t½ is the time for half of them to go, which gives

t½ = ln2/λ

For radium-226, the isotope the Curies isolated in bulk, the IAEA nuclear data service lists a half-life of 1600 ± 7 years (IAEA^[11]). So

λ = 0.693 / 1600 y ≈ 4.33 × 10⁻⁴ per year

Over a century, e^(−0.0433) ≈ 0.958: about 95.8% of a radium sample is still radium after 100 years. It takes about 5300 years (3.32 half-lives) to fall to a tenth. Radioactive compounds went on to become “important as sources of radiation in both scientific experiments and in the field of medicine” (NobelPrize.org^[5]).

So who really discovered radioactivity?

Becquerel discovered the phenomenon. Marie Curie discovered that it is atomic, named it, and with Pierre Curie discovered polonium and radium. Rutherford and Soddy discovered what it is: elements transforming. All of them were recognised by the Nobel committees. If you want more stories like this, read Cecilia Payne and what stars are made of, or browse Physicist Stickers collection and The Physicists sticker sheet.

FAQ

Did Marie Curie discover radioactivity?

No. Henri Becquerel discovered it in 1896. She coined the word and showed it is an atomic property.

Which elements did Marie Curie discover?

Polonium and radium, both announced with Pierre Curie in 1898.

How many Nobel Prizes did Marie Curie win?

Two: a quarter share of the 1903 Physics prize and the whole 1911 Chemistry prize.

References

1. NobelPrize.org, Henri Becquerel – Facts (Nobel Prize in Physics 1903). https://www.nobelprize.org/prizes/physics/1903/becquerel/facts/
2. American Institute of Physics, “Marie Curie and the Science of Radioactivity: Research Breakthroughs (1897–1904)”, part 1, X-rays and Uranium Rays (online exhibit, 2000). https://history.aip.org/exhibits/curie/resbr1.htm
3. NobelPrize.org, Marie Curie – Facts (Nobel Prize in Physics 1903). https://www.nobelprize.org/prizes/physics/1903/marie-curie/facts/
4. American Institute of Physics, “Marie Curie and the Science of Radioactivity: Research Breakthroughs (1897–1904)”, part 2, The Discovery of Polonium and Radium (online exhibit, 2000). https://history.aip.org/exhibits/curie/resbr2.htm
5. NobelPrize.org, Marie Curie – Facts (Nobel Prize in Chemistry 1911). https://www.nobelprize.org/prizes/chemistry/1911/marie-curie/facts/
6. American Institute of Physics, “Marie Curie and the Science of Radioactivity: Research Breakthroughs (1897–1904)”, part 3 (online exhibit, 2000). https://history.aip.org/exhibits/curie/resbr3.htm
7. NobelPrize.org, The Nobel Prize in Physics 1903 (Henri Becquerel, Pierre Curie, Marie Curie). https://www.nobelprize.org/prizes/physics/1903/summary/
8. NobelPrize.org, The Nobel Prize in Chemistry 1911 (Marie Curie). https://www.nobelprize.org/prizes/chemistry/1911/summary/
9. NobelPrize.org, Ernest Rutherford – Facts (Nobel Prize in Chemistry 1908). https://www.nobelprize.org/prizes/chemistry/1908/rutherford/facts/
10. NobelPrize.org, Frederick Soddy – Facts (Nobel Prize in Chemistry 1921). https://www.nobelprize.org/prizes/chemistry/1921/soddy/facts/
11. IAEA Nuclear Data Section, LiveChart of Nuclides data service: Ra-226 ground state (ENSDF). https://www-nds.iaea.org/relnsd/v1/data?fields=ground_states&nuclides=226ra

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.