Radioactivity: a property of the atom

Henri Becquerel had found in 1896 that uranium salts fog photographic plates. Few people followed it up, because X-rays seemed more exciting. Marie Curie chose the neglected rays for her doctorate in late 1897 precisely because almost nothing had been written about them.

Instead of photographs she measured the tiny electric current the rays allow to flow through air. Compound after compound gave the same answer: the activity was proportional to the amount of uranium and indifferent to whether the uranium was a metal, an oxide or a salt, wet or dry, warm or cold, lit or dark. As she wrote, "the emission of the rays is an atomic property of the uranium."

It was a bold conclusion at a time when atoms were thought to be indivisible and unchanging, and it hinted that something happens inside them. It also gave her a tool: any extra activity in a mineral meant an extra element. "It was necessary at this point to find a new term to define this new property of matter," she wrote. "I proposed the word radioactivity." The word first appeared in print, as *radio-active*, in the polonium note of July 1898. In her 1911 Nobel lecture she restated that the idea of radioactivity as an atomic property was hers.