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322
Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter.
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460
Democritus First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle.
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Period: 460 to
Atom Timeline
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John Dalton Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
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Henri Becquerel while studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.
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JJ Thomson discovers electron
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Marie and Pierre Curie Studied radioactivity which turned out to be the spontaneous splitting of unstable atoms.
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Robert Millikan discoverd that the elcectron is negatively charged
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Ernest Rutherford established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.
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Niels Bohr: Bohr Model, The correct theory of the atom is called quantum mechanics; the Bohr Model is an approximation to quantum mechanics that has the virtue of being much simpler.
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Louis de Broglie suggested that an electron moves with a wavelength that completes a circle, which would explain the different energy levels as Bohr’s model did but would also explain the wave diffraction pattern
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James Chadwick discovered a previously unknown particle in the atomic nucleus. This particle became known as the neutron because of its lack of electric charge