The Nobel Prize in Chemistry 1911 was awarded to Marie Curie "in recognition of her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element".
 

In 1896, Becquerel observed that the compounds of the element uranium gave off rays which had the property of acting on photographic plates and of making air conduct electricity. A little later, it was noticed that the compounds of another element, thorium, already discovered by Berzelius, possess similar properties. For the discovery and investigation of this radiation, called uranic or Becquerel rays, the Academy of Sciences awarded the Nobel Prize for Physics in 1903 to Henri Becquerel and Pierre and Marie Curie jointly.
 

Radium, the only one of these two elements which it has been possible to isolate in the pure state so far, resembles the metal barium in its chemical properties, and is distinguished by a very characteristic spectrum. The discovery of radium and polonium, an even more radioactive element, has brought in its train the discovery of a great many other radioactive elements with longer or shorter life-spans, by which our field of knowledge in chemistry and our understanding concerning the nature of matter have been considerably extended.