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He proposed the law of multiple proportions, which lead to the atom theory. He thought that all matter is made of atoms.
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He identified the negatively charged electrons. He also found protons.
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He conducted the gold foil experiment which supported his nuclear model. He shot alpha particles at gold foil to see where they deflected to.
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He created a model of the structure of a atom. Showing it being a positively charged nucleus surrounded by electrons. Which are traveling in circular orbit and are defined by their energy levels.
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He was able to show electrons don't move around the nucleus in orbits, but in clouds where their positions are uncertain. His model is still considered widely accurate.