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J.J. Thompson discovered electrons and that electrons are a piece of every atom of each element. -
Rutherford's team observed, in an experiment shooting alpha particles into gold foil, that the mass of atoms is located at their center. The parts in the center held all the mass and had positive charges while the rest of the atom remained mostly empty space. -
Mosely, in his research into x-rays, finds that the rays behave like light and can be diffracted. Instead of being diffracted by a crystal to create a rainbow, the x-rays create a symmetrical pattern of spots on a photographic plate. He finds that the atoms in the crystal diffracting the rays are arranged in a specific pattern resulting in the pattern of spots on the plate. He discovers that each element diffracts x-rays differently, leading to the discovery of new elements. -
Chadwick, part of Rutherford's research team, discovers that atoms have a part called the neutron found in the nucleus of the atom. This meant that other atoms do not have to go against the positive charge of the protons and can go through the nucleus. -
Fermi fires neutrons at uranium atoms. Pieces of the uranium atoms chip off and change the mass of the atom. He saw that sometimes the neutrons are absorbed by the nucleus of the uranium atom and then change into a proton. -
McMillan validates Fermi's finding and proves that the atom left after splitting the uranium atom becomes a completely new element.