Atomic Theory

492-375 BC: Empedocles

100

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Proposed the idea that all matter is made from Air, Fire, Water and Earth.

400 B.C. Democritus

100

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Democritus believed that if matter was divided to smaller pieces there would be a point in which it could not be divided further. He said matter was made of indivisible (not dividible) particles called atoms

0-1700s The time of Alchemy

Dec 31, 750

While many advances were made in the area of chemistry during this time no significant advancement was made to the ideas of atoms and their makeup

1704 Isaac Newton

Aug 22, 1704

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Isaac Newton theorized a mechanical universe with small, solid masses in motion.

1803 John Dalton

Dec 31, 1803

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John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined togethe

1874 G.J. Stoney

Dec 31, 1874

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G.J. Stoney theorized that electricity was comprised of negative particles he called electrons.

1897 J.J. Thomson

Dec 31, 1897

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Used experiments with Cathode Ray Tubes to determine the existance of the electron as well as the fact that electrons were negatively charged.
Later proposed one of the first atomic models known as the "Plum Pudding Model"

1911 Ernest Rutherford

Dec 31, 1911

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Rutherford did work in his lab to confirm Thomson's theory of the atom. His work however broke down the plum-pudding model.

The results of this experiment gave Rutherford the means to arrive at two conclusions: one, an atom was much more than just empty space and scattered electrons (J.J. Thomson model argued), and two, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the

1922 Niels Bohr

Dec 31, 1922

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Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.

1920 - present Modern Theory-Quantum Mechanics

Dec 31, 1975

There is no one scientist mentioned here because modern atomic theory is the collective work of many, many scientists! These include but are not limited to Max Planck, Albert Einstein, Werner Heisenberg, Erwin Schrodinger and others
In this model electrons are not located in discrete orbits, as hypothesized in the Bohr model, but instead occupy a hazier region, called an orbital. An orbital indicates a probable location of the electrons in an atom instead of a definite path.