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Democritus, of Greece, theorized that everything is made of atoms, which are indestructible and too small to see, and differ in size, shape, and arrangement.
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John Dalton, of the UK, theorized that all matter is made of atoms, which are indivisible. He suggested that atoms of a given element are identical in mass and properties, compounds are combinations of two or more different types of atoms, and a chemical reaction is a rearrangement of atoms.
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Dmitri Mendeleev, of Russia, organized the a table of all known elements by each element’s atomic weights, which tried to make sense of the relationships between all of the elements. The table later became organized by atomic number (number of protons) as suggested by British Henry Moseley in 1914.
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JJ Thomson, of England, discovered the existence of electrons and theorized that there must be the particles that are positively charged, because atoms have a neutral charge, not a negative charge. Both these particles were examples of subatomic particles.
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J.J. Thomson, of England, connected a cathode ray tube to electricity and saw that he could see particles moving around, which were attracted to the positive end of a magnet- meaning the particles were negatively charged. This led to the creation of his Plum Pudding model of an atom, where electrons were randomly scattered throughout.
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Ernest Rutherford, of New Zealand, created his Gold Foil experiment where he shot positive particles at a screen and saw some scattering of the particles. He then realized this scattering was because some particles were hitting a positively charged part of the atom, the nucleus, and were being repelled by its positive charge.
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Niels Bohr, of Denmark, theorized that electrons traveled in orbits around the nucleus, with the outer orbits determining the atom’s chemical properties.
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Ernest Rutherford, of New Zealand, bombarded hydrogen gas with particles to knock the nuclei out of the atoms, and realized that nitrogen could be made into oxygen by bombarding it with particles, with a second product that looked like the hydrogen nuclei, meaning that hydrogen nuclei were a piece of all nuclei - a new particle, the proton.
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Francis Aston, of England, invented the mass spectograph, which electromagnetically separated different isotopes of elements based on their slightly different masses. This provided proof of the existence of different isotopes of elements.
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Erwin Schrödinger, of Austria, theorized at the University of Zurich that electrons could be treated as waves of matter, which allows measurement of electrons’ energy levels. Schrödinger created a wave equation for this, known as the Quantum Mechanical Model.
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James Chadwick, of England, shot radiation at hydrogen-containing paraffin and observed that the paraffin's protons were scattered by radiation particles similarly sized to protons-- neutrons.
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Murray Gell-Mann, of the United States, theorized the Eightfold Way theory at the California Institute of Technology, which made sense of about 100 different particles in the nucleus. The theory suggested that all of the particles are made of smaller particles, called quarks.
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