Marie and Pierre Curie (jan 1, 1898 – jan 1, 1899)
Marie and Pierre Curie are primarily known for their 1898 discovery of the elements radium and polonium; said elements were discovered in the process of the Curies experimenting with radiation. Whilst studying Henri Becquerel's discovery of radiation, Marie wanted to know if there were other atoms that emitted the same form of energy (radioactive). Her discovery proved that radioactive energy is based on the origin of the atom rather than how the atoms are arranged in a molecule.
Added to timeline:
Atomic Theory project