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Looking for Layman's Physical Explanation for energy level in atoms

  1. Dec 6, 2008 #1
    I'm looking for a simple physical explanation of the discrete energy level in atoms, so I can provide a brief explanation to interested laypeople (not to mention just to help myself understand better). Does anybody want to take a crack at it? Or am I asking for the impossible?
     
  2. jcsd
  3. Dec 6, 2008 #2
    The simplest model used to describe the energy levels in atoms is the Bohr model. You can find this by googling it or in elementary physics or chemistry texts. Be advised that the model sacrifices accuracy for simplicity...in general, as the models become more and more accurate they become more and more complex.
     
  4. Dec 6, 2008 #3

    Bohr claimed energy levels are quantized in atom. But I think Bohr's argument is wrong and circular, since he assumed that angular momentum is quantized in the first place, without any justification
     
    Last edited: Dec 6, 2008
  5. Dec 6, 2008 #4

    dx

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    The justification is that it fits experiments.
     
  6. Dec 6, 2008 #5
    At that time, experiments justify the quantization of energy, but not the quantization of the electron's orbital angular momentum. In fact, the electron's orbital angular momentum is zero in the ground state of hydrogen atom.
    Bohr model fall apart when the angular momentum is zero
     
    Last edited: Dec 6, 2008
  7. Dec 6, 2008 #6

    jtbell

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    If your audience is acquainted with musical instruments and the concept of harmonics for a vibrating string or air column or drumhead, you can make an analogy with that. Just like vibrating strings etc. have discrete standing-wave modes which produce the harmonics, the QM wave function of a bound system has discrete standing-wave modes which produce the energy levels.
     
  8. Dec 6, 2008 #7
    Yup, without this sort of justification quantum theory would be screwed.

    The Bohr model has its shortcomings, but contains many of the basic concepts of quantum theory that still are present in the more advanced theories in modified forms (it introduced the very concept of quantized angular momentum that was carried over into later theories). The main failure of this model is that it restricts the movement of the electrons around the nucleus too rigidly compared to the models that followed.

    If you really don't like it you can take a shot at a "layperson version" of the Shrodinger Equation with standing waves on a rope or guitar strings and such, but I personally wouldn't take this route unless my audience understood the Bohr model first (that's because I am not a big fan of "failing").
     
  9. Dec 7, 2008 #8
    I think you miss the point. Bohr model fail miserably if you apply it to simple harmonic oscillator. Since angular momentum is zero in SHO but energy is still quantized, so angular momentum can Not be the reason for energy quantization
     
  10. Dec 7, 2008 #9
    I thought your goal was to give a simple introduction to the energy levels in atoms? Maybe I did miss the point...
     
  11. Dec 7, 2008 #10

    ZapperZ

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    But you are applying apples to oranges here. You can't use the SHO potential to an atom. That's just the wrong potential, so the argument that it doesn't work because the angular momentum is zero is moot.

    I do not advocate using the Bohr model. However, there are many who still view them as an effective teaching method and argued that if done properly, may in fact serves its purpose. See, for example, McKagan et al., Phys. Rev. Sp. Topics - Phys. Ed. Res., v.4, p.10103 (2008).

    Zz.
     
  12. Dec 7, 2008 #11
    Pauli once remarked sadly 'That's not right. It's not even wrong, your theory make no prediction'". In a certain sense, the prediction of Bohr theory is very limited. It can apply only to one electron atom like hydrogen. It's just a collection of some ad hoc rules of quantization
     
  13. Dec 7, 2008 #12

    ZapperZ

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    That's fine. Like I said, I don't advocate using it. But don't use the SHO as the reason.

    Zz.
     
  14. Dec 8, 2008 #13
    Why not? Don't tell me what to do or what not to do!!
    If you know the answer, why don't you spell it out for us?
    My guess is that you probably don't have an answer
     
    Last edited: Dec 8, 2008
  15. Dec 8, 2008 #14

    atyy

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    Isn't the Bohr model related to Somerfeld quantization which gives the SHO correctly?
    http://en.wikipedia.org/wiki/Old_quantum_theory
    http://people.ccmr.cornell.edu/~muchomas/8.04/Lecs/lec_bohr-sommerfeld/notes.html

    I know the theory has been superseded - but Schroedinger was superseded by the Dirac wave equation which was superseded by QFT - so there is always another level, and Bohr was historically important and the idea of discrete energy levels being determined by boundary conditions on the wave solution carries over to Schroedinger.
     
    Last edited: Dec 8, 2008
  16. Dec 8, 2008 #15
    In all seriousness, it depends upon what you mean by "physical explanation". Please elaborate upon what you think that means to the intended lay person audience?
     
  17. Dec 9, 2008 #16

    ZapperZ

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    What did you think that I did in Post #10 already?!

    I clearly explained why using the SHO potential is clearly irrelevant here in arguing why the Bohr model doesn't work! And did you even READ the link that I gave? How much more spoon-feeding do you need?

    Zz.
     
  18. Dec 9, 2008 #17

    epenguin

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    Let me follow this veering off the topic to the virtues or otherwise of teaching the Bohr model

    Only by the very high standards of fundamental physics. In any other science the Bohr model would count as an incredibly compact condensate into a very few postulates.

    Surely never has Science owed so much to so short-lived a theory!? To anyone who says it is only of historical interest I would answer yes it is of little interest if Science teaching is only about teaching technicians how to get right answers.

    One advantage of having relatively elementary students go through proofs of Bohr type calculations is simply because they can!

    Quote from Bohm
    "Where the theory was in error, however, it was seldom very much so; hence it was clearly on the right track."
     
  19. Dec 9, 2008 #18

    ZapperZ

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    There is a problem though, in the sense that if this is not done properly (see the article I cited), you could end up with giving the students the wrong information that tend to live with them for the rest of their lives. Unfortunately, and anyone who has been on PF for any considerable amount of time can testify, this wrong idea about the atom has persisted for a large portion of the population. So essentially we are teaching them about "Santa Claus" (which, after all at that time, had its purposes), and no one told them when they grew up and it really doesn't exist.

    We don't teach students about the Caloric theory and all the other now-erroneous ideas of physics, regardless of their historical significance or whether they were on "the right track". While the Bohr model can be useful in a limited range of cases, it creates way too much issues and problems that I don't see it as being worthwhile. The benefits here do not outweigh the problems. This is especially true if the instructor does not follow through and later on, correct the Bohr model and not leave the students hanging with the wrong picture.

    Zz.
     
  20. Dec 9, 2008 #19

    epenguin

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    For didactics I can see arguments on both sides, and in the teaching as in history it would have to be a brief passage.

    If you are teaching the majority of general students, not destined to become professional physicists, about what Science is and its development, then the passages are almost if not equally as important as the arrival points?
     
  21. Dec 9, 2008 #20

    ZapperZ

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    Just to be clear, I am less concerned about teaching the Bohr model to physics students than I am to students that will not pursue a physics degree. Physics majors, sooner or later, will see the error in the Bohr model. Non-physics students do not, and if we stop at teaching them with the Bohr model, they are left "hanging" as I've said before, and stopped at an erroneous idea about the atom. That, to me, is unacceptable. No one corrected them that there's no Santa Claus.

    Zz.
     
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