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Democritus said that all matter is made up small indestructible units called atoms.
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Aristotle didn't believe in atomic theory, but that four elements, Earth, Fire, Water, and Air made up the earth.
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Proust created his Law of Definite Proportions which states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created.
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Lavoisier found that mass is conserved in a chemical reaction. The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction. This led to the law of conservation of matter, which states that matter is conserved in a chemical reaction.
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He determined the charge to mass ratio of electrons.
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Rutherford discovered alpha, beta, and gamma rays in radiation. He also discovered nucleus is heavy.
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He determined the size of the charge on an electron. He also determined that there was a smallest unit charge, or that charge is quantized.
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Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
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He contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behavior of elementary particles and atoms.
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Schrodinger developed an “Electron Cloud Model."
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Chadwick discovered neutrons which are particles whose mass was close to that of a proton.