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Democritus and Kanada develop idea that all matter is composed of atomos
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Dalton formulated modern version of atomic theory; created five principles that govermed atomic theory. Performed experiments to show that matter consisted of elementary particles. Imagined atoms to be like tiny marbles, or rigid spheres, that cannot be subdivided, created, or destroyed.
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G.J. Stoney proposed that electricity was made of negative particles called electrons
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Thomson discovered the electron, working with cathode ray tubes and proved electrons exist and have a negative charge.
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Nagoaka proposes a model of the atom that has a large sphere in the center with a positive charge. The model shows electrons revolving around sphere-like planets that reolve around the Sun. This has been called the "Saturnian" or "Planetary System Model."
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Rutherford discovers, using his gold foil experiment, that the atom is mostly empty space with a small postiveily charged nucleus containing most of the mass, orbited by the negatively charged particles (electrons) containing low mass. He called the postiveily charged particles in the nucleus, protons. Also, he hypothesized the existence of neutrons.
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Niels Bohr develops first succesful model of atom, building upon Rutherford's basic conception. In this model, electrons are not randomly located around the nucleus. The model shows electrons moving on specific layers, or shells
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Moseley discovers the charges on nuclei of most atoms. He discovers the idea of atomic number, the number or protons in the nucleus,. This work is used to re-organize the periodic table based upon atomic number, rather than atomic mass
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Louis de Broglie hypothesizes that electrons in Bohr's model are confined to discrete orbits because they have the properties of standing waves. His theory becomes known as the Principle of Complementarity
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Erwin Schrodinger takes de Broglie's theory a step further, viewing electrons as cotinuous clouds and introduces "wave mechanics" as the mathematical model of an atom. His work contributes to the view of an atom as an electron cloud. In this model, clouds consist of a probability distribution map which determines the most probable location of an electron
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Paul Dirac hypothesizes the existence of positively charged electrons
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James Chadwick discovers the neutron, a particle having about the same mass as a proton, but with no electrical charge. Existence of the neutron explains why atoms are slightly heavier than the total mass of their protons and electrons
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Carl Anderson, experimenting with cosmic rays, confirms Dirac's hypothesis, and discovers the anti-electron. Physicists call it the Positron
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Enrico Fermi conducts the first controlled chain reaction, releasing energy from the atom's nucleus
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Physicists find the antiproton and, later, the antineutron. This allows for the existence of anti-atoms, a true existence of antimatter
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Many new paricles have been discovered during the 1950's
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Murray Gell-Mann and Yuval Ne'man independently proposes a method for classifying all particles then known.This method becomes known as the Eightfold Way
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Gell-Mann proposes the existence of a new level of elementary particles and calls them quarks
