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Proposed that all matter is made up of tiny, indivisible particles called "atomos."

Believed atoms differed in shape and size but were otherwise unchangeable.

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Rejected Democritus' idea of atoms.

Believed matter was made of four elements: Earth, Water, Air, and Fire.

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Proposed the first modern atomic theory.

Stated that atoms are indivisible and combine in whole-number ratios to form compounds.

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Discovered the electron using the cathode ray tube experiment.

Proposed the "Plum Pudding Model," where negatively charged electrons were embedded in a positively charged sphere.

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Conducted the gold foil experiment.

Discovered that atoms have a dense, positively charged nucleus surrounded by empty space.

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Proposed that electrons orbit the nucleus in fixed energy levels (Bohr Model).

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Developed the quantum mechanical model.

Described electron locations as probability clouds rather than fixed orbits.

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Discovered the neutron, explaining atomic mass discrepancies.