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Old May 27th 2008, 07:19 AM   #1
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Atomic Structure

Q: In 1997 we celebrated the centenary of JJ Thomson’s discovery of the electron. Thomson pictured the atom as being like a currant bun, with electron ‘currants’ embedded in a positively charged ‘bun’. Give a short account of three ways in which our model of matter on a subatomic scale has changed since Thomson’s discovery. For each, state clearly how the model changed, and write a few sentences outlining one piece of evidence that played a part in bringing about the change.

How would I answer it? (I'm not too good at answering theoretical question, better with numbers ) Thanks in advance.
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Old May 27th 2008, 10:06 AM   #2
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Originally Posted by Air View Post
Q: In 1997 we celebrated the centenary of JJ Thomson’s discovery of the electron. Thomson pictured the atom as being like a currant bun, with electron ‘currants’ embedded in a positively charged ‘bun’. Give a short account of three ways in which our model of matter on a subatomic scale has changed since Thomson’s discovery. For each, state clearly how the model changed, and write a few sentences outlining one piece of evidence that played a part in bringing about the change.

How would I answer it? (I'm not too good at answering theoretical question, better with numbers ) Thanks in advance.
We now use a Quantum Mechanical model to model the atom. (But I'm wondering if you are supposed to be talking about the Bohr model? Some of those historical links at the bottom might be quite useful.)

1. The current (not currant, take note!) model of the atom involves a very small, very dense nucleus containing both protons and neutral particles (neutrons) which were missing from the Thompson model.

2. An additional feature of the current model relating to the first point is that it predicts the existence of a new force of nature: the strong nuclear force. Hey, something has to be holding all those positive charges together. This force (naturally) was missing from Thompson's model.

3. The Quantum model is based on probabilities, whereas most of the Science in Thompson's era was based on "determinism." Classical Physics does not take into account the Uncertainty Principle, the wave nature of matter, etc.

4. The Thompson model did not explain the light spectra that is observed when passing light through a tube of hot (or cool) atomic gases. This feature is explained by the current model and predicts that the electrons fall into discrete energy states within the atom.

There's four off the top of my head. I'm sure you can find more, so you should have a pretty good number to make a selection of three that you can discuss. If you would like some more elaboration on any of this, just let me know.

-Dan
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Old May 27th 2008, 11:24 AM   #3
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From My chem notes:

Discovery of the nucleus (alpha particles fired at a gold sheet. some repelled straight
back, indicating dense positive charge) - Rutherford, around 1911

The idea of electron shells (From analyzing ionisation energies) - Bohr, around 1913

Subshells, probabilities associated with the positive of an electron - Schrodinger, late 1920's

Discovery of the neutron (missing mass of atoms - strong force) - Chadwick, around 1932
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