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The idea that things were made of small particles was first thought. at the time Democritus called them Atomos. They weren't advance theories, just that if you cut something enough, eventually you are left with an un cuttable particle.
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Believed that everything was made of 4 elements, earth, fire, air, water. this was believed for a very long time and took over the Atomos
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Discovered that matter not only had a negative charge but also a positive charge
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Discovered that atoms contained small negatively charged particles. he called them corpuscles which later became known as electrons
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Ernest Rutherford completed an experiment where he discovered that an atom must have a positive charge in one specific area (nucleus)
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predicted locations of electrons within the atom and built upon the belief that most mass was within the nucleus.
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Worked further on quantum mechanics and found it was impossible to find where electrons would be and how fast they would be moving
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Quantum theory and cloud theory are discovered showing that electrons were not particle or waves but has qualities of both