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If the bohr model of the atom is correct

  1. Jun 20, 2012 #1
    so imagine a hyper-classical scenario.

    A single point elecrron is orbiting a point nucleus. No other charged objects are nearby.

    There is no em radiation from the electron. The electron orbits the nucleus in perfect circular motion. Zero orbital precession, zero uncertainty in the electron's momentum and position.

    Now if i suddenly disturbed the electron by giving it a push toward the nucleus, what would happen?

    Certainly its orbit will be disturbed. But could the electron fly out of orbit? Would it go into a new stable orbit, like an elliptical or parabolic orbit?
     
    Last edited: Jun 20, 2012
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  3. Jun 20, 2012 #2

    mathman

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    In terms of energy levels, giving it a push of sufficient energy would put the electron into a higher level, or possibly kick it out of the atom altogether.

    The electron levels do not have a geometric shape - in that regard the original Bohr picture is incorrect.
     
  4. Jun 20, 2012 #3

    Nugatory

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    Well, the Bohr model is not correct (at least based on what we know) which makes this question rather meaningless. The electron isn't a point particle, doesn't orbit around the nucleus so it doesn't have any circular orbits to be knocked out of, nor elliptical orbits to be knocked into.

    Now, there is a related question that you might be asking... If I have a small but macroscopic object, like say a grain of sand, floating in a vacuum, and it has a positive electrical charge... And then take an even smaller object, a fleck of dust with about 1/1000th the mass of my grain of sand and give it a negative charge... Could I arrange to put that fleck of dust in a circular orbit around the grain of sand, and can I then disturb it into an elliptical orbit? (The hyperbolic and parabolic trajectories generally are not considered stable orbits).

    If that's the question you're asking, I'm going to answer it (with great trepidation because I suspect that you're about to misuse that answer in some fashion): Yes.

    As a historical note - Bohr himself would have severely chastised you for asking the original form of the question.
     
  5. Jun 20, 2012 #4
    well, ever since i first saw the bohr model in grade 3, i felt that the reason it could not exist was because the electrons would be easily knocked out of orbit. Just wanted to know if that was a valid assumption.


    The bohr model cant explain covalent bonds at all, but it actually does a pretty good job with spectral lines.

    Also, why would bohr chatise me?
     
    Last edited: Jun 20, 2012
  6. Jun 21, 2012 #5

    vanhees71

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    I cannot agree more! I think, the Bohr model shouldn't be taught anymore at any level of education (be it in high school or in universities). It's misleading right from the beginning! There are no orbits for electrons around atomic nuclei. There cannot be bound states of a charged classical particle bound to another charged particle within classical electrodynamics etc. etc.

    It's not so bad that Bohr's model is wrong. This could be true for any other theory as well. Physics is never finished but always subject to be changed by new observations. In fact, that's what brings science forward: We learn by disproving models and sometimes even theories by observation and experiment. Why I hate the Bohr model so much from a didactic point of view is, because it gives totally wrong intuition about the inner workings of the atom and other "microscopic" phenomena. It is difficult enough to build the right intuition when learning quantum theory without first getting used to totally wrong notions!

    Bohr has been very sceptical about his model already when he invented it in 1912 or so, but at that time it was the only way the physicists could think about the "quantum hypothesis" with regard to bound electrons in atoms, and in fact the Bohr model has been an important predecessor model to develop modern quantum theory, which in its present form has been found by Heisenberg in his famous paper worked out on the island of Helgoland in 1925 and then very quickly has been worked out by Born, Jordan, and Heisenberg himself in terms of "matrix mechanics". At about the same time also Schrödinger came up with his wave-mechanics formulation in a series of some of the most marvelous papers ever written on the subject, however lacking the probabilisitic interpretation, which has been given as a footnote in another famous paper by Born in 1926. Last but not least in a totally independent work also Dirac came up with quantum theory in its abstract form in 1925 in another of the most marvelous papers on the subject (anyway, all papers of Dirac's are just brilliant, particularly compared to the very enigmating writing of Heisenberg or Bohr; the same holds for Pauli, who has given the first calculation of the nonrelativistic hydrogen spectrum within Matrix mechanics, who has been of utmost clarity in writing. The same is true for Born's papers.).

    The breakthrough for Bohr, and his really important contribution, came with Heisenberg's work on the famous uncertainty formulation in 1927. The first idea has not yet been correct, and this has been fixed by Heisenberg and Bohr in one of the painful discussion sessions, for which Bohr was famous (or infamous, depending on the "victim" ;-)).

    On the other hand Bohr is guilty of inventing the "Copenhagen interpretation" with his "cut" between a classical and a quantum world (there is no such sharp cut known on physical grounds yet; classical physics is an emergent phenomenon from the point of view of quantum theory and valid as an approximation, when decoherence is efficiently at work, which is the case under almost all circumstances (but not always!) concerning macroscopic situations) and, even worse, the collapse of the state, instead of strictly sticking to Born's probability interpretation, nowadays known as Minimal Statistical Interpretation. See

    Ballentine, L. E. The Statistical Interpretation of Quantum Mechanics. Rev. Mod. Phys. 42 (1970), 358–381;

    Ballentine, L. E. Quantum Mechanics. World Scientific, Singapore, New Jersey, London, Hong Kong, 1998.
     
  7. Jun 21, 2012 #6
    About that last sentence: yes exactly. But then, do you also think that classical ("Galilean") mechanics should not be taught because it's inaccurate and misleading as well? As long as it's presented as just a model -or even a "toy" model- I think that it's perfectly fine, including the educational aspect of hammering down that we only work with models and cannot say for sure what's really going on.
    I won't believe you if you claim that you can prove with 100% certainty that there are no orbits for electrons around atomic nuclei. Therefore I also doubt that you can prove that it creates a "totally wrong intuition". Different models create different intuitions, with each different issues. Let's agree that we would like known issues to be mentioned right at the start.
    I asked you about that interpretation elsewhere; thanks for providing two references here. :smile: But regretfully those aren't papers that I can download... :grumpy:
    And don't some recent papers claim that statistical interpretations have been disproved? I think that there is even a live topic on it in this forum...
     
  8. Jun 21, 2012 #7
    so do you think my assumption about the orbits a valid "disproof" of the bohr model?
     
  9. Jun 21, 2012 #8

    mfb

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    Classical mechanics is a good model for all everyday phenomena. And the concept of objects with a certain position, momentum, mass, ... is fine for relativistic dynamics, too. You just have to avoid quantum effects.

    Bohr model can postdict energy levels in hydrogen-like atoms. Nothing else. It cannot even give some deeper explanation, as you have to insert the allowed energy levels (as "multiple of de-Brogle wavelengths") by hand. And it is misused quite frequently, as it looks so simple and is so wrong at the same time.
     
  10. Jun 21, 2012 #9

    Nugatory

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    At the risk of putting words in vanhees71's mouth..... I think you're misreading his point.

    Galilean mechanics is contained within relativistic mechanics; the two agree when v<<c. In principle we could teach SR to high school students first, and then show them how classical mechanics emerges for small values of v, without ever misleading or having to walk back any statement that we've made. Of course we'd lose 99% or more of our students on the way, so we teach forward instead: first we teach classical mechanics; then an explanation of why E&M suggests that we shouldn't expect classical mechanics to work well at sufficiently high speeds; then how to compatibly extend classical mechanics into relativistic mechanics.

    We can't do the same with the Bohr model. If you start with classical physics and work forwards through the Bohr model, you end up at a dead end. And although it is possible to work backward from the QM model of the atom to conclude that it contains the the Bohr model as a subset, the exercise has neither predictive nor pedagogical value.

    So vanhees71 is raising an interesting question - why do we mention the Bohr model at any level of education? It doesn't contribute to understanding, it has to be unlearned (not extended, as with classical mechanics, but just plain unlearned) if the student is going to progress further. So why do we give it any air time at all?

    I believe that it's because a general science curriculum is supposed to include the history and philosophy of science as well as just the results. It's at least as important to teach how science operates and how it arrived at its current understanding of the world as it is to teach the details of that understanding. Where the curricula fall down is that they fail to clearly identify the Bohr atom as a dead end, one of the many good models that didn't stand the test of time.
     
  11. Jun 21, 2012 #10

    vanhees71

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    Yes, that's precisely the point. Of course we have to learn classical mechanics and electromagnetics before we can turn to quantum mechanics and quantum field theory. I don't think that one can understand quantum theory without a good fundamental knowledge of classical physics.

    This is the more justified, because classical physics is a successful model with a limited applicability range, which is given by quantum theory. Within this validity range it's describing successfully a lot of phenomena from very few assumptions (mostly the structure of spacetime and its symmetries).

    Contrary to that, Bohr's model is an ad-hoc construction leading by chance to the right energy levels of the hydrogen atom. It cannot describe successfully any other atom's energy levels without additional ad hoc assumptions. This very disappointing state of affairs led the physicists in the early 20th century to think harder about a better model, and the outcome is quantum theory, of which yet no limitation of validity has been found despite very concise experimental tests regarding its very foundations.
     
  12. Jun 21, 2012 #11
    With that argumentation I can agree. :smile:
     
  13. Jun 21, 2012 #12
    Not really: you should be able to demonstrate that the model is unstable when that happens (with electrons being knocked out, as you put it). I'm sure people calculate these things in the past, but now I don't have a reference ready.
     
  14. Jun 21, 2012 #13
    Would it be more correct to describe the electrons as rings/spheres around the nucleus rather than point particles?
     
    Last edited: Jun 21, 2012
  15. Jun 23, 2012 #14

    mfb

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    Something like a cloud would be better - they are usually called orbitals, and always three-dimensional.
     
  16. Jun 23, 2012 #15

    vanhees71

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    The cloud picture is close to be right, but you have to interpret the cloud correctly. It's not to be understood as if the electron is a continuous entity smeared as a "cloud" over the whole space.

    The cloud rather represents the probability distribution for the position of the electron around the nucleus. This is the basics of the Born Rule, which lies at the heart of the Minimal Statistical Interpretation.
     
  17. Jun 25, 2012 #16
    I skimmed the wiki on orbitals. It seems that all orbitals are symmetrical around some axis, and I'm wondering why that is.

    I have developed a pet theory that indicates something like orbitals and energy levels should exist when dealing with electrons and nuclei. The theory is the product of two fairly simple concepts, bremsstrahlung and conservation of angular momentum. For those not aware, bremsstrahlung is radiation that is emitted by accelerating (or decelerating) electrical charges, while angular momentum has to be conserved in all systems.

    If you take a ring (torus) that is electrically charged, and spin it like a wheel in a zero-resistance space, then, even though each point on the ring is accelerating indefinitely, it cannot emit bremsstrahlung except for some discreet wavelengths, due to conservation of angular momentum. It is similar to a bohr atom I guess, and I am wondering whether it could be related to atomic orbitals using some maths or something.
     
    Last edited: Jun 25, 2012
  18. Jun 26, 2012 #17
    I just found a very recent and serious looking paper that develops such concepts (without mention of orbitals though) on Arxiv by Poelz, 1206.0620v1. However, this forum isn't meant to discuss such theories except if they were published in physics journals.
     
  19. Jun 26, 2012 #18

    mfb

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    You can derive it with the Schroedinger equation in the case of a simple hydrogen-like atom.
     
  20. Jun 27, 2012 #19
    sorry mate but i think heisenberg can resolve that problem of momentum conservation.
    Plus i dont think you should discuss your own theories here.
     
  21. Jun 27, 2012 #20
    The Bohr model also works very well for Rydberg atoms with high orbital angular momentum. When the expectation value of orbital radius and angular momentum of an atomic electron is high, the electron effectively acts like a classical particle.
    Also, the semiclassical approximation works rather well if one includes a Lorentz invariant background of electromagnetic radiation. This approximation of quantum mechanics is called stochastic electrodynamics (SED). The picture of the Bohr atom that you were given is a valid visualization for SED.
    The Lorentz invariant background in SED is corresponds to the virtual particle background used in quantum electrodynamics (QED). Photons are not a fundamental particle in SED. SED looks slightly magical, but is more intuitive than QED. Let me repeat: SED is an approximation of QED.
    The Bohr atom without the background has too many failings to be useful even as an approximation. However, SED accurately most of the phenomena described in chemistry classes. SED fails only in nonlinear interactions that involve more than two photons. Since chemical reactions don't usually involve high photon densities, the SED model does include the formation of covalent bonds.
    I conjecture that the Bohr atom picture could be kept in introductory chemistry courses with a superficial description of SED theory. Teachers could say up front that there is a more accurate physics model called QED. However, I think that students would be more comfortable with SED explanations than quantum mechanical explanations because SED explanations don't include photons. Electromagnetic waves are simply waves, not wavicles. Therefore, the student doesn't have to throw away what he learns in classical optics.
    Therefore, I think the Bohr atom picture may still have a place in chemistry education provided one includes the SED approximation.
     
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