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According to Democritus' atomic theory, the universe and all matter obey the following principles: Everything is composed of “atoms”, which are physically, but not geometrically, indivisible.
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John Dalton hypothosised that elements were made up of samll paritcles called atoms. Atoms of the same element are the same, atoms of different elements are not the same. He also discovered that atoms combine in simple whole number ratios to create compounds
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Thomson determined that all matter is made up of tiny particles that are much smaller than atoms. He originally called these particles 'corpuscles,' although they are now called electrons. This discovery contradicted the current theory that the atom was the smallest unit.
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Rutherford found out that nearly the total mass of an atom is in a nucleus. In so doing, he created the nuclear model, a discovery that started nuclear physics and ultimately brought upon invention of the atom bomb.
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Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another.
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Assuming that matter (e.g., electrons) could be regarded as both particles and waves, in 1926 Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms.
