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370
Democritus 370 B.C.
Democtritus believed that there is a certain point where a substance can no longer be divided into more parts. He called this the "atomos" which means "indivisible". -
400
Aristotle 400 B.C.
Aristotle proposed his theory that all matter was made of Earth, Air, Water, and Fire. -
Period: 500 to
Development of the Atom
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John Dalton
Dalton developed five main points to the Atomic Theory:
1. Elements made of small particles called atoms
2. Atoms of one element are equal in size, and mass but differ in other elements.
3. Atoms cannot be subdivided, destroyed, or created.
4. Atoms of different elements combine to make chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.
These points set the basis of all further atomic research. -
Henri Becquerel
Becquerel discovered radioactivity on accident while performing other experiments. He was further investigating phosphorescence in uranium salt. He wrapped a flourescent substance in photographic plates and black material to prepare for an experiment using bright sunlight. But before he could perform this experiment, the photographic plates had already been disposed revealing the image of the object used. -
J.J. Thomson
Thomson proposed that the fundamental unit was 1000 times smaller than an atom, suggesting the sub-atomic particle called the electron. -
Marie & Pierre Curie
The couple began researching Science together, but after Pierre's death, Marie continued to study radiology and discovered Polonium and Radium. -
Robert Millikan
Robert Millikan worked on an oil-drop experiment where they measured the charge of one single electron. He worked off of JJ Thomson's "charge-to-mass" ratio to determine the actual charge and mass values. -
Ernest Rutherford
Rutherford theorized that the positive charge of an atom is concentrated in the nucleus. he also discovered two different radioactive substances in 1908: Thorium and Uranium. -
Niels Bohr
Bohr created a model of the atom in which the electrons orbit around the nucleus. He also created the basis for the "quantum theory" by coming up with the idea that an electron could drop from a high-energy orbit to a low-energy orbit to create a "light quantum of energy". -
Quantum Model Created
The Quantum model is a possible location of the atom in an element. The Quantum Model also includes the nucleus containing both nuetrons and protons. -
James Chadwick
Sir James Chadwick discovered the particle with a lack of electric charge called the Nuetron, He discovered that the Nuetron is located in the nucleus.
