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400
Democritus
Democritus stated that all matter was composed of atoms and could no longer be divided.He acknowledged the idea that all atoms were composed of the same primary matter, but with different sizes, shapes, and weights. -
400
Aristotle
Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. He believed in the four elements of air, earth, water and fire. -
Issac Newton proposed a mechanical universe with small solid masses in motion.
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Dalton
John Dalton developed the first coherent atomic theory. He stated that all matter is composed of atoms, that all atoms of an element are identical, during a chemical reaction atoms are neither made nor destroyed, and atoms always combine in whole number multiples. -
Benjamin Franklin originated the idea that an elementary unit of charge should exist
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First periodic table established
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Sir WIlliams Crookes
Developed a better vacuum pump that allowed him to produce cathode-ray tubes with a smaller residual gas pressure. -
G.J. Stoney proposed that electricity was made of discrete negative particles he called electrons
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Rutherford
Ernest Rutherford proposed the nuclear atom as the result of the gold-foil experiment. -
Henri Becquerel discovered that a sample of uranium was able to expose a photographic plate even when the sample and plate were separated by black paper.
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J.J. Thomson
Thomson proposed that an atom was composed of a spherical ball of positive charge with "corpuscles" of negative charge imbedded in it. The corpuscles would later become known as electrons. -
The Curies
The Curies discovered another radioactive element in pitchblende. They named it `polonium’ in honor of Marie Curie’s native homeland. -
Albert Einstein published the famous equation E=mc^2
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Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen.
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Millikan
Robert Millikan determined the unit charge of the electron . -
Henry Moseley
Henry Moseley discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. -
Henry Moseley
Moseley proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements. -
Bohr
Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. -
Aston discovered the existence of isotopes through the use of a mass spectrograph.
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Wolfgang Pauli predicted that an electron should spin (kind of like a top) while it orbits around the nucleus.
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Paul Dirac produced equations which predicted an unthinkable thing at the time- a positive charged electron.
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Schrodinger viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.
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The problem of the extra nuclear mass was solved
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James Chadwick finally discover the neutron.
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Murray Gell-Mann and Yuval Ne'man independently proposed a method for classifying all the particles then known. The method became known as the Eightfold Way.
