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History and Models of the Atom

Explore the evolution of atomic models from John Dalton's "Billiard Ball" model to Niels Bohr's orbiting electrons and the current Quantum Mechanical model. Discover how scientific evidence has shaped our understanding of the atom over time.

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History and Models of the Atom

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  1. History and Models of the Atom Pages 89 - 97

  2. The “Billiard Ball” Model • proposed by John Dalton in 1804 • this theory proposed that matter was composed of small, spherical particles • but evidence was later gathered that matter was composed of even smaller bits

  3. in the 1900s evidence was discovered regarding charges: • atoms have positive and negative parts • charges interact: • as a result, revisions to Dalton’s model had to be made

  4. positive (evenly distributed) “dough” part negative “chocolate” “Chocolate Chip Cookie” Model • using available data on the atom, J.J. Thompson came up with the idea of having charges embedded with Dalton’s Billiard Balls note: this model kept Dalton’s key ideas intact

  5. Nuclear Model • Ernest Rutherford discovered a huge flaw in the previous concept of the atom during his now famous gold foil experiment for experiment setup see figure 4-11 (page 95)

  6. Nuclear Model • if previous models were correct alpha particles would have passed straight through the the gold

  7. - - - - - + Nuclear Model • Rutherford found that most (99%) of the alpha particles that he shot at the gold went straight through

  8. Nuclear Model • from these experiments Rutherford concluded that the atom had a dense positive core, with the rest composed of mostly empty space with the occasional negatively charged electron note: this model completely changed the definition of atom

  9. Nuclear Model • Rutherford could not explain why the nucleus did not explode due to repulsion • it was not until 1932 that the neutron was discovered, which helps counteract the positive repulsion

  10. negative electrons 3 positive protons Bohr Model • Niels Bohr proposed that electrons revolve around the central positive nucleus (like planets in the solar system)

  11. no energy level in between steps Bohr Model • Bohr also suggested that the electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized)

  12. Quantum Mechanical Model • the current understanding of the atom is based on Quantum Mechanics • this model sees the electrons not as individual particles, but as behaving like a cloud - the electron can be “anywhere” in a certain energy level

  13. electrons can be found anywhere in these “shells” Quantum Mechanical Model note: the electrons are still quantized no electrons can be found here

  14. Note About Chem 11 • most things we do can be explained using Dalton’s and/or Bohr’s model • the Quantum Mechanical model, although most accurate, is complex even at a university level (conceptually and mathematically) and will not be discussed here

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