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1808 atomic theory: elements are made up of atoms, atoms of an element are identical, chemical compounds are formed when atoms combine, compounds have the same relative types and numbers of atoms, and chemical reactions reorganize, not change, atoms. 


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Used cathoderay experiments to produce electrons and determine that all atoms must contain electrons and thus assumed the existence of protons as well. Proposed the plum pludding model, in which electrons were embedded in a diffuse positive charge. 


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Shot alpha particles at gold foil, and the deflections showed that atoms must have a large positive center – the nuclear atom. 


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Oil drop experiments determined the magnitude of the electron charge and the mass of the electron (9.11 x 1031 kg). 


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Developed the quantum model of the hydrogen atom, with electrons moving along circular, leveled orbits. As the electron is brought closer to the nucleus, energy is released. Does not work with other elements, now fundamentally incorrect. 


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Postulated that energy can only be gained or lost in wholenumber multiples of Planck’s constant, the value 6.626 x 1034 Js. This showed that energy can occur only in units called quantums, and cannot be emitted in unlimited amounts as previously believed. 


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Experimented and came up with de Broglie’s equation (wavelength = h/mv, where h is Planck’s constant and v is the velocity of a particle) to determine that particulate matter has wavelike properties, shown by diffraction. 


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Worked out the quantum (wave) mechanical model for the hydrogen atom in which the electron was assumed to behave as a standing wave. His equation gives solutions of wave functions, or orbitals, in which electrons move. 


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Developed the Heisenberg uncertainty principle: the more accurately we know a particle’s position, the less accurately we can know its momentum and vice versa. For an electron, we cannot know its exact motion, so electrons cannot move in welldefined (Bohr) orbits. 


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Postulated that in a given atom, no two electrons can have the same set of four quantum numbers (n, l, ml, and ms). This Pauli exclusion principle therefore states that an orbital can hold only two electrons, and they must have opposite spins (ms values). 

