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Discovery: Proposed that matter is composed of indivisible particles called atoms.

Picture: An illustration of the solid sphere model or Dalton's atomic theory.

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Discovery: Discovered the electron and proposed the "plum pudding" model of the atom.

Picture: Plum pudding model (electrons in a positive "pudding").

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Discovery: Discovered the nucleus through the gold foil experiment.

Picture: The nuclear model of the atom (a dense nucleus with electrons orbiting).

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Discovery: Proposed that electrons orbit the nucleus in distinct energy levels.

Picture: Bohr’s model with electrons in defined orbits around the nucleus.

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Discovery: Discovered the neutron, which explained the mass of the atom.

Picture: A diagram of the neutron within the nucleus of the atom.