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Democritus believed the universe was made of empty space and tiny bits of “stuff”
-This “stuff” was so small that they could no longer be divided into smaller pieces.
-These tiny pieces he called atoms.

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He believed that matter was made up of atoms too small to be seen by the naked eye. He also believed that each element was made up of one type of element.
His theory was called the "Atomic Theory of Matter." His model was called Billiard Ball Model."

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Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight.

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He discovered the negatively charged particles called electrons. He came up with the "Plum Pudding Model."

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The Curies conducted experiment on uranium and concluded that the rays stayed the same no matter the condition of the actual uranium. She then theorized that this was because of the atom's structure.

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Millikan succeeded in determining the magnitude of the electron's charge.

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Rutherford discovered the nucleus in 1911.

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In 1913 Bohr came up with the "Bohr Model" consisting of a small, dense nucleus surrounded by orbiting electrons. (Also called the Solar System Model)

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The Electron Cloud Model shows the theory that there is a 'cloud' with negatively charged electrons.

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Chadwick proved the existence of neurons.