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Aristotle believed in the four elements of earth, wind, water, and fire. He also believed that a portion of matter could be cut in half infinitely many times. (364 B.C.)
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Democritus was the first person to propose the existence of an ultimate particle. He described this particle using the word "atomos". He made this proposition in 420 B.C.
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Jospeph Proust proposed the law of Constant Composition which states that samples of a pure compond always contain the sam elements in the same mass proportion
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John Dalton proposed the Law of Multiple Proportions. This law led to the development of the atomic theory. He pictured the atom as a small indestuctible particle with no internal structure.
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Thomson identified the negatively charged electron. He concluded that the electron was a component of all matter. He also proposed the "plum pudding" model of the atom.
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Henri Bequerel discovererd natural radioactivity in elements.
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Max Planck developed the concept of quantum mechanics. His This concept served as the foundation for all modern physics and most atomic models.
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Millikan dtermined the unit charge of the electron. His discovery allowed for the calculation of the electron and all positively charged atoms.
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Ernest Rutherford published an atomic theory in which the atom has a central positive nucleus surrounded by negative orbiting electrons.
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Bohr applied the quantum theroy to Rutherford's atomic structure and discovered a method of calculating the possible energy levels of stationary electrons. He also discovered that an emission of light occurs when an electron moves into a differnet energy level orbit.
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Broglie proposed that electrons could behave as waves under certain conditions.
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Erwin Shrodinger developed the Shrodinger equation. This equation can be used to predict the properties reactivities of all atoms and molecules. His work led to the atomic cloud model.
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Werner Heisneberg discovered the Heisenberg Uncertainty Principle which states that the momentum and postion of a particle cannot be simultaneously measured with high precision.
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James Chadwick confirmed the existence of the neutron.
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