Cobra Effect · Chemistry
The ore that gave off too many rays
How rays that were too strong to explain led to two new elements.
7 cards, read aloud in 3:27, with a test and sources.
In 1896 Henri Becquerel found that uranium gives off invisible rays.
He left uranium salts on a photographic plate wrapped in black paper. When the plate was developed, it was fogged where the uranium had been. The rays came from the uranium itself, with no sunlight needed to start them. Few scientists paid much attention. Marie Curie chose the rays for her doctoral research.
Marie Curie measured the rays, and one ore gave off far more than it should.
She used a sensitive electrical instrument invented by Pierre Curie and his brother Jacques. She found that the strength of the rays depended on how much uranium was present, whatever it was mixed with. She named this radioactivity. But pitchblende, a uranium ore, was more radioactive than its uranium alone could explain. She reasoned that the ore must hold something else, something new.
In 1898 the Curies announced two new elements, polonium and radium.
Pierre joined Marie in the hunt. They split pitchblende into its chemical parts, and followed the radioactivity from one part to the next. In July they announced polonium, named after Marie’s homeland, Poland. In December they announced radium, from the Latin word for a ray.
To convince other chemists, Marie set out to make radium pure.
A new element was only fully accepted once it could be weighed. The Curies worked in a leaky, poorly ventilated shed in Paris. Marie later wrote that she sometimes spent a whole day stirring a boiling mass with an iron rod nearly as big as herself. The work went on for nearly four years.
Tons of ore gave only a tenth of a gram of radium.
In 1902 Marie finally had a tenth of a gram of pure radium chloride. It was enough to measure the atomic weight of radium, and to prove the element was real. Radium salts glowed faintly in the dark, and Pierre liked to carry a sample in his waistcoat pocket to show friends. Nobody yet understood how harmful the rays could be.
Marie Curie became the first person to win two Nobel Prizes.
In 1903 she shared the Physics prize with Pierre and Becquerel, and became the first woman to win a Nobel Prize. In 1906 Pierre was killed when a horse-drawn wagon ran him down in a Paris street. Marie took over his post, and became the first woman to teach at the Sorbonne. In 1911 she won the Chemistry prize alone, for polonium and radium.
So when a result is bigger than it should be, ask what else is there.
Marie Curie trusted her measurements when pitchblende gave off more rays than its uranium could explain. That surplus led to two new elements, and to the idea that the rays come from inside atoms. The rays also harmed her. She died in 1934 of a blood disease caused by years of exposure to radiation. Her notebooks are still radioactive, and readers who ask to see them must accept the risk in writing.
Sources
- Marie and Pierre Curie and the discovery of polonium and radium, NobelPrize.org. The measurements, the shed and the two new elements, told in detail.
- December 1898: The Curies discover radium, APS News. A short history of the announcement, and what followed.
- Marie Curie, Britannica. Her life, both Nobel Prizes, and the radiation that killed her.
Nearby ideas
- The table that predicted missing elements. How a table of the elements left gaps, and the gaps were filled as predicted.
- There is light on both sides of the rainbow. How thermometers, silver salts and a glowing screen found light we cannot see.
- The bounce that revealed the nucleus. How a few particles bouncing back off gold foil revealed the nucleus.
- Fertiliser made from thin air. How chemists learned to pull fertiliser out of the air, and what else it made.
- The ozone hole and the treaty that worked. How a warning about spray-can chemicals led to a treaty every country signed.
- Weighing everything changed chemistry. How weighing everything before and after overturned an old idea about burning.