History and Models of the Atom Pages 89

History and Models of the Atom Pages 89

History and Models of the Atom Pages 89 - 97 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 in the 1900s evidence was discovered regarding charges: atoms have positive and negative parts charges interact:

as a result, revisions to Daltons model had to be made Chocolate Chip Cookie Model using available data on the atom, J.J. Thompson came up with the idea of having charges embedded with Daltons Billiard Balls

positive (evenly distributed) dough part negative chocolate

note: this model kept Daltons key ideas intact 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) Nuclear Model if previous models were correct alpha particles would have passed straight through the the gold Nuclear Model

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

- 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 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 Bohr Model Niels Bohr proposed that electrons revolve around the central positive nucleus (like planets in the solar system) negative electrons

3 positive protons 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) no energy level in between steps

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

Quantum Mechanical Model electrons can be found anywhere in these shells note: the electrons are still quantized no electrons can

be found here Note About Chem 11 most things we do can be explained using Daltons and/or Bohrs 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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