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Atomic history day #2 1

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Ernest Rutherford discovered the structure of the atom through his gold foil experiment in 1909. He fired alpha particles at a thin gold foil and discovered that some particles were deflected at very large angles or reflected straight back, contrary to expectations. This showed that the mass of an atom is concentrated in a very small, dense central nucleus. Niels Bohr then built on this work by proposing in 1913 that electrons orbit the nucleus in fixed orbits, establishing the early nuclear model of the atom.

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Ernest Rutherford (1871-1937) • Learned physics in J.J. Thomson’ lab. • Noticed that ‘alpha’ particles were sometime deflected by something in the air. • Gold-foil experiment
Rutherford ‘Scattering’ • In 1909 Rutherford undertook a series of experiments • He fired (alpha) particles at a very thin sample of gold foil • According to the Thomson model the particles would only be slightly deflected • Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source Lead collimator Gold foil particle source
Rutherford’s Apparatus beam of alpha particles radioactive substance fluorescent screen circular - ZnS coated gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
What he expected… California WEB
Because he thought the mass was evenly distributed in the atom. - - - - -
Because, he thought the mass was evenly - - distributed in the atom - - - - - - - - - - - - - - - - - - - - - - -
What he got…
Density and the Atom • Since most of the particles went through, the atom was mostly empty. • Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. • Small volume and big mass = big density • This small dense positive area is the nucleus California WEB
The Rutherford Atom n+ Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
Niels Bohr • In the Bohr Model (1913) the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. • They are not confined to a planar orbit like the planets are.
Bohr Model Planetary model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus.

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Atomic history day #2 1

  • 1. Ernest Rutherford (1871-1937) • Learned physics in J.J. Thomson’ lab. • Noticed that ‘alpha’ particles were sometime deflected by something in the air. • Gold-foil experiment
  • 2. Rutherford ‘Scattering’ • In 1909 Rutherford undertook a series of experiments • He fired (alpha) particles at a very thin sample of gold foil • According to the Thomson model the particles would only be slightly deflected • Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source Lead collimator Gold foil particle source
  • 3. Rutherford’s Apparatus beam of alpha particles radioactive substance fluorescent screen circular - ZnS coated gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
  • 5. Because he thought the mass was evenly distributed in the atom. - - - - -
  • 6. Because, he thought the mass was evenly - - distributed in the atom - - - - - - - - - - - - - - - - - - - - - - -
  • 8. Density and the Atom • Since most of the particles went through, the atom was mostly empty. • Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. • Small volume and big mass = big density • This small dense positive area is the nucleus California WEB
  • 9. The Rutherford Atom n+ Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
  • 10. Niels Bohr • In the Bohr Model (1913) the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. • They are not confined to a planar orbit like the planets are.
  • 11. Bohr Model Planetary model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus.