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400
Atomic Theory Introduced
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) -
400
All substances made of 4 elements
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. -
Dalton's Atomic Theory
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. -
Plum Pudding Model
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. -
Radioactivity
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. -
Radioactivity
Marie and Pierre Curie proved that there was radioactivity in atoms. This helped shape the understanding of the atom. -
Charge of Electron
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. -
Discovery of the Nucleous
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. -
Quantum Theory of Atomic Structure
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. -
Discovery of the Nuetron
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. -
Quantum Model
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.
