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Democritus discovered the atomic theory through philosophical reasoning and "thought experiments". He proposed that all matter was composed of tiny, indivisible particles, which he called atomos.
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He proposed that matter is composed of indivisible and indestructible atoms, each element having unique atoms, and that atoms combine in simple whole-number ratios to form compounds.
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He discovered the electron and the proposal of the "plum pudding" model of the atom in 1897, which suggested that atoms are composed of negatively charged electrons within in a sphere of positive charge.
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He discovered the atomic nucleus through the gold foil experiment, which led to the development of the nuclear model of the atom.
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He proposed the Bohr model which suggests that electrons move around the nucleus in fixed, stable orbits, with specific energy levels. This idea helps explain how atoms absorb and emit light.
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He discovered the neutron, a neutral subatomic particle with a mass like a proton. It sits in the nucleus alongside protons, completing our understanding of the atom's structure.