In 1896 Henri Becquerel had noticed that uranium salts, with no external cause, spontaneously gave off invisible rays. No one knew the source of this radiation. Polish-born Marie Curie (1867–1934), working in Paris, chose this strange phenomenon as her doctoral subject — and turned it from a footnote into an epoch-making science.

Marie measured the intensity of the radiation precisely and reached a striking conclusion: the radiation was independent of the chemical form of the matter or of external conditions; it depended only on the amount of uranium within. This meant the radiation came not from molecules but from the atom itself. She named the phenomenon "radioactivity." Moreover, seeing that some minerals radiated far more strongly than pure uranium, she predicted they must contain still-unknown, far more active elements.

With her husband Pierre, she processed tonnes of pitchblende ore by hand for years in a primitive shed to isolate these elements. In 1898 they announced two new ones: "polonium," in honour of her homeland Poland, and "radium," for its intense radiation. Obtaining radium pure in visible quantity took years more.

Radioactivity showed that within the atom — thought until then indivisible and unchanging — there was a source of energy, an inner structure; Rutherford's nucleus and all of nuclear physics were born from this discovery. Marie Curie became the first, and for long the only, person to win two Nobel Prizes (physics and chemistry). Because the danger of radiation was not yet known, she worked for years unprotected; the anaemia that caused her death was most likely the result of that exposure. Her notebooks are still radioactive today.