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In 400 B.C., ancient Greek philosopher Democritus first introduced the notion of the atom. He claimed it was the smallest part of matter and called this an "atomos". "Democritus performed no experiments" to prove his hypothesis. (http://abyss.uoregon.edu)
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In 400 B.C., Aristotle introduced his theory that all substances are made of fire, earth, water and wind. This model was adopted to explain what substances were made of. It is believed Aristotle never experimented to draw his conclusions, rather it was a popular thought.
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In 1780, John Dalton became the "father of atomic theory" because of his atomic theory. He hypothesized that 1. atoms make up atoms 2. atoms of the same element are identical 3. atoms of different elements are different 4. atoms of different elements react to form compounds and 5. atoms cannot be created or destroyed- only changed. Dalton's experiments with gases led him to his theory.
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In the 1890s, J.J. Thompson found the negative particle in an atom called the "electron". He found this by using the Cathode Ray Tube experiment.
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In 1896, Henri Becquerel accidentilly contributed to the atomic theory. He was studying the effects of x-rays on photographic film he saw there was chemicals randomly decomposing and giving off penetrating rays. He had discovered radiation.
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Marie and Pierre Curie proved that there was radioactivity in atoms. This helped shape the understanding of the atom.
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In 1909, Robert Millikan discovered the chage of a nelectron to be negative. He did this by using the oil-drop experiment. Because of his contribution to the atomic theory, scientists now know the charge of an electron.
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In the 1910s, Ernest Rutherford found a solid part in the atom. This disproved Thomsons pudding model. His gold foil experiment lead him to his discovery of the nucleous. This discovery leads to great advancements in modern chemistry.
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In 1913, based off of Rutherford's work, Niels Bohr changed our understanding of how an atom works. He stated that the atom emits electromagnetic raditatoin only when an electron changed energy levels. His work contributed greatly to scientists today's work on quantum theory.
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In 1932, James Chadwick proved the existence of the nuetron. He proved the nuetron to exist by using beryllium ray experiments. Chadwick's discovery contributed to science today by proving the existence of the nuetron.
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This is the most recent model of the atom. It is still used by scientists today. It is based on quantum mechanics. It was discovered by Niels Bohr.
