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883 BCE
Aristotle
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Period: 883 BCE to 322 BCE
Aristotle
contribution to atomic theory -
aristotle believed that all things were composed of matter.
model name/desciption -
He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
460 BCE
Democritus
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Period: 460 BCE to 370 BCE
Democritus
Contribution to atomic theory-
Everything is composed of atoms which are physically but not genetically invisible.
Model name/description-
Simple round sphere without electrons, neutrons, or protons. His contribution helped people with understanding the idea of an atom, and helped other scientists further look into the science of the atom and its genetic makeup -
Dalton
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Period: to
Dalton
contribution to the atomic theory-
proposed that all material was composed of atoms invisible and indestructible building blocks while all atoms of an element were identical different elements had atoms of different size and mass
model name/description-
He defined an atom to be a ball like structure, as the concepts of atomic nucleus and electrons were unknown at the time. -
Thomson
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Period: to
Thomson
contribution to atomic theory-
discovered the electron by experimenting with a crookes or cathode ray tube he demonstrated that cathode rays were negatively charged . in addition he also studied positively charged atoms in neon gas
model name/description-
In Thomson's model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged “plums” surrounded by positively charged “pudding”. -
Rutherford
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Period: to
Rutherford
contribution to atomic theory-
Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.
model name/description-
Rutherford's model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set, predictable paths. -
Bohr
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Period: to
Bohr
contribution to atomic theory-
Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted
model name/description-
The Bohr diagram, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar to structure of the Solar System, but with attraction provided by electrostatic forces rather than gravity. -
Chadwick
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Period: to
Chadwick
contribution to atomic theory -
chadwick was the first one to discover the neutron.
model description/name -James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.
