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He had an idea of an atom, meaning a piece of matter that was not able to be divided

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He created the very first atomic theory. All substances are made of atoms.

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He discovered negatively charged particles inside the atom (electrons).

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He proposed that in the center of the atom is positively charged part called the nucleus. He discovered the protons and nucleus.

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He improved on Rutherford's model. He proposed that electrons move around the nucleus in certain paths called energy levels.

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He proposed that electrons do not travel in definite paths around the nucleus.