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Democritus' idea was that all matter is formed of different tiny particles and their properties also determine matter's properties.
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Aristotle actually did not believe in the atomic theory and otherwise believed not in atoms but that everything on Earth was made up of the four elements, water, air, earth, and fire.
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He discovered that the total mass in a chemical reaction of products and reactants is always the same which eventually led to the theory of the law of conservation of mass.
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Proust had discovered that the same substance will always contain the same elements at the same ratios no matter where it is found.
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Dalton came up with a modern theory about the atom that all matter is made up of atoms that are indivisible and indestructible.
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Faraday contributed a lot towards the understanding of electricity by discovering the idea that atoms had an electrical component.
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Becquerel discovered the theory of radioactivity which showed the atom is not indivisible making scientists change ideas about atomic structure.
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Thomson did experiments and used his plum pudding model to help him find out that all atoms contain very small negatively charged particles which are electrons.
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Planck made his quantum theory which proved that through electromagnetic radiation the smallest possible form of energy that can be emitted or absorbed is known as quantum.
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Marie and Pierre Curie found out about atomic decay and successfully isolated radium salts from the mineral pitchblende. They also used their knowledge about radioactivity to discover the elements radium and polonium.
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Einstein was able to use math to prove that atoms existed. He then was able to prove that a liquid's molecules are always moving randomly.
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Through the Rutherford model he discovered that most of the atomic matter is actually condensed into a small volume in the center of the atom, which is the nucleus.
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Bohr discovered lots about the atom through his Bohr model which shows that electrons orbit around the nucleus in orbits with set size and energy. This means the electrons do not crash into the nucleus but instead are in specific energy levels.
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Millikan used his oil drop experiment to find out that the electric charge is conveyed based on the charge of the electron.
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Chadwick discovered a new uncharged particle inside the core and he called it the neutron.