History of Atomic Structure. Aristotle Democritus.
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Transcript of History of Atomic Structure. Aristotle Democritus.
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History of Atomic Structure
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Aristotle
Democritus
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Ancient Philosophy• Who: Aristotle, Democritus
• When: More than 2000 years ago 460 BC
• Where: Greece
• What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.
• Why: Aristotle and Democritus used observation and inference to explain the existence of everything.
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• Any experimental support for ideas?
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Alchemists• Who: European Scientists
• When: 800 – 900 years ago
• Where: Europe
• What: Their work developed into what is now modern chemistry.
• Why: Trying to change ordinary materials into gold.
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Alchemic Symbols
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John Dalton
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Particle Theory• Who: John Dalton
• When: 1766-1844
• Where: England, school teacher
• What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.
• Why: Building on the ideas of Democritus in ancient Greece.
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4 Part Atomic Theory• 1. Elements are composed of tiny indivisible
particles (atoms).
Does this part hold true today?
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• 2. Atoms of the same element are identical
and different from other atoms.
Does this part hold true today?
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3. Atoms can physically mix and chemically combine in whole number ratios.
• Does this part hold true today?
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4. In chemical reactions, atoms separate, join and rearrange.
Atoms cannot be changed into other atoms.
Does this part hold true today?
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• Did Dalton have experimental support for his theory?
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Dalton’s Solid Sphere Model of the Atom
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Discovery of Electrons• Who: J. J. Thompson
• When: 1897
• Where: England, physicist
• What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged.
• Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.
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J. J. Thompson
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• Thomson’s Cathode Ray Tube
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• Thomson’s Plum Pudding Model of the Atom
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• Who: Robert Milliken
• When: 1868-1953
• Where: United States, physicist
• What: Conducted an experiment to determine the quantity of charge on an electron, 1-, in his famous oil drop experiment
• Why: He knew that atoms had electrons, wanted to determine charge
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• Robert Milliken
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Atomic Structure I• Who: Eugene Goldstein
• When: 1850-1930
• Where: England
• What: Conducted an experiment with a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles- protons.
• Why: He concluded that they were composed of positive particles- protons
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Atomic Structure I• Who: Ernest Rutherford
• When: 1911
• Where: England
• What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.
• Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.
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Ernest Rutherford
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• Who: James Chadwick
• When: 1932
• Where: England, physicist
• What: confirmed existence of the neutron
• Neutrons have no charge and have a
mass nearly equal to that of a proton
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Atomic Structure II• Who: Niels Bohr
• When: 1913
• Where: England
• What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.
• Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.
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Niels Bohr
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Electron Cloud Model• Electrons travel around the nucleus in
random orbits.
• Scientists cannot predict where they will be at any given moment.
• Electrons travel so fast, they appear to form a “cloud” around the nucleus.
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Electron Cloud Model