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-Alchemists 500BC- develop the theory—that all metals are composed of mercury and sulphur and that it is possible to change base metals into gold.
-Democritus 400 B.C. atomic theory posited that all matter is made up small indestructible units he called atoms.
-Aristotle 300BC He also believed that everything was a combination of the four elements: earth, fire, water, air. His theory was that a mass of incomprehensible size was everywhere; he called this 'hyle'. There was no separate 'particles' for each material, it was all one.
-Joseph Priestly 1733-1804 (discovered oxygen) isolated several gases and showed how they could be produced from other subsances. These early chemists began to scientifically discuss the substances they were isolating. Discussions lead to new experiments, which guided the way to further discoveries.
J.J. Thomson 1897, determined the charge to mass ratio of electrons.
-Niels Bohr 1922 proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
-Robert Brown 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason.
James Chadwick 1932 James was most famous for his discovery of the neutron in an atom.
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