In 1803, British chemist John Dalton described atoms as the unique constituents of the chemical elements. All atoms of one element, say hydrogen, were alike and weighed the same, Dalton determined. But all hydrogen atoms were fundamentally different from atoms of the other elements. It wasn't until the turn of this century that experiments hinted at the power trapped inside the atom. In 1896, French physicist Antoine Henri Becquerel discovered radioactivity by exposing photographic film to uranium.
Radiation is the energy given off by atoms.
A handful of unstable heavy metals emit energy when their nuclei disintegrate. In 1903, Marie and Pierre Curie realized that uranium ore contained more radioactivity than could be accounted for by the uranium itself. From tons of ore, they isolated small amounts of two highly radioactive new elements - radium and polonium. They shared the Nobel Prize in physics in 1903. Marie Curie won the chemistry Nobel in 1911 for describing the new elements' properties.