Atom
In 1937, German chemist Otto Hahn became the first person to make nuclear fission in a laboratory. He discovered this by chance when shooting neutrons at a uranium atom, hoping to make a new isotope. However, instead of a new isotope, the uranium changed into a barium atom, a smaller atom than uranium. Hahn had "broken" the uranium atom. This was the world's first recorded nuclear fission reaction. This discovery led to the creation of the atomic bomb and nuclear power, where fission happens over and over again, creating a chain reaction.