Uranium
Antoine Henri Becquerel
The sun stayed hidden for several days and Becquerel couldn't continue with his experiment. But he decided to develop his film anyway. He was astonished to find a black spot right in the middle, just where the uranium crystal had been! That meant that even without any sunlight, the crystal had given off rays of its own! Another mystery!
Becquerel tested several compounds that contained uranium and found that they all gave off similar rays. Why?
Marie Curie
One of the scientists who heard about Becquerel's work was a young Polish woman working in Paris. Her name was Marie Curie (Cu-REE).
Madame Curie wondered if uranium were the only element to give off rays. With the help of her husband, Pierre, she began to test every known element for rays. She decided to call this ability to give off rays radioactivity.
After many tests, Madame Curie found that the element thorium was also radioactive. So, the Curies reasoned, if there were two radioactive elements, there were probably more. And they continued to search.