jan 1, 1898 - MARIE CURIE (1867-1934 CE) date of discovery (1898 CE) Atom is composed of particles and the nuclei of heavy, unstable elements lose mass (decay) and give off energy (radioactivity). Discovered unknown elements polonium and radium and deduced that radioactivity does not depend on how atoms are arranged into molecules, but rather that it originates within the atoms themselves.
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jan 1, 1898
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~ 127 years ago