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495 BCE
Empedocles
- Believed everything was made from four elements (water, fire, air, earth)
- Did not believe in a pure element; All objects were a mixture of the four elements
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400 BCE
Democritus
- Believed atoms were small, invisible things made up of the four elements
- Believed there could be "pure" elements
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350 BCE
Aristotle
- Didn't believe atoms existed - Based on the four elements
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John Dalton
- Recognized the existence of atoms and elements - Developed a relative atomic weight scale - Law of multiple proportions - Proposed the atomic theory in 1803
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Henry Becquerel
- Discovered radioactivity in atoms which helped future scientists to perfect the atomic model
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JJ Thompson
- Positive and negative charges were evenly distributed in the atom
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Planck
- Proposed the Quantum Theory - led to an understanding of atom levels - his discoveries helped future scientists advance the atomic model
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Rutherford Experiment
- Shot Alpha particles through gold foil - Positive charges were concentrated in the nucleus with negative charges placed around the nucleus - Atoms are mostly empty space - Atoms have a neutral charge
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Neils Bohr
- Bohr Atomic Model was similar to Rutherfords, but the electron orbit was based on energy levels - Electrons change energy levels -Electrons can be thought of as waves - Model still used today
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Ernest Marsden
- Discovered the proton - Found the connection between the atomic number and the nuclear charge
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James Chadwick
- Discovered the neutron