In 1896, French scientist Antoine Henri Becquerel discovered radioactivity which was an early contribution to atomic theory. He discovered this phenomenon while experimenting with uranium and a photographic plate.
Becquerel began his experiment by exposing a crystal that contained uranium to sunlight. After the crystal had soaked up some sunshine, Antoine Becquerel placed it on a photographic plate. The uranium crystal imprinted its image on to the photographic plate, leading Becquerel to the conclusion that the uranium was releasing the absorbed energy of the sun in the form of an x-ray. In the absence of light, however, the uranium still imprinted its image on to the photographic plate. Becquerel was surprised, since there was no obvious source of energy involved. He assumed the image was caused by a spontaneous emission from the uranium, but he had in fact discovered radioactivity.
Following up on Becquerel's discovery, Pierre and Marie Curie began experimenting with uranium and the concept of radioactivity. In 1903, the Curies and Becquerel were jointly awarded the Nobel Prize in physics for the discovery of radioactivity and their research. Marie Curie also discovered two new radioactive elements which were named polonium and radium. She was awarded a second Nobel Prize in 1911 for her findings, become the first person to ever be awarded two Nobel Prizes.