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Democritus thought that if you kept cutting a stone it would be so small that it would be indivisible. He called the small piece of matter "atomos"
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Aristotle believed that everything was a combination of water, fire, earth, and air. There were no separate particles for each material, it was all one.
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He proposed the Combustion Theory which was based on sound mass measurements. He also proposed the Law of Conversation of Mass which represents the beginning of modern chemistry.
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Proved that the relative quantities of any given pure chemical compounds elements remain invariant, regardless of the compounds force. This is known as the law of definite proportions.
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Dalton proposed an atomic theory with spherical solid atoms based upon measurable properties of mass.
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Used a CRT to experimentally determine the charge of mass ratio of an electron. Studied canal rays and found they were associated with proton H+
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Rutherford studied radiations emitted from uranium and thorium and named them alpha and beta.
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Bhor proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined equations. Electrons should move around the nucleus but only in prescribed orbits.
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Created the theory of quantum mechanics. For this theory and the applications of it which resulted especially in the discovery of allotropic forms of hydrogen.
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Used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.
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Using alpha particles discovered a neutral atomic particle with a mass close to a proton Thus discovered the neutron.
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Classified particles and their interactions. He proposed that the observed particles are comprised of smaller building blocks.