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Nucleus at the center?

  1. Feb 1, 2016 #1
    How do we know that the nucleus is at the center of the atom?

    I know about the Geiger/Marsden experiment and also of its results. They observed a few α-particles scattering almost at 180°. Hence they concluded that there must exist a high density, positively charged "center" in the atom. But on what basis was it concluded that this "center" must be at the center of the atom? Why not on one side or in random locations for random atoms?
     
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  3. Feb 1, 2016 #2

    mfb

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    You cannot conclude that based on this experiment alone, but there is no mechanism that could lead to an asymmetry. The negatively charged electrons are attracted by the positively charged nucleus, why should they be more on one side compared to the other, and what would determine the orientation of this favored side?
    Later experiments allowed to directly calculate and test the electron orbitals, of course, so this is a purely historic question.
     
  4. Feb 1, 2016 #3

    Vanadium 50

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    How do you know the sun is in the center of the solar system? (Not an unrelated question, believe it or not)
     
  5. Feb 2, 2016 #4
    Is it? :)
     
  6. Feb 2, 2016 #5

    Vanadium 50

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    If you don't want my help, fine.
     
  7. Feb 2, 2016 #6
    Don't get hurt man! I didn't mean to hurt you. Im sorry if I did.

    What I meant was- " IS the sun in the center of the solar system?" I mean, who even proved that convincingly? Add to that elliptical orbits and defining a "center" becomes very messy. So we cannot decide if the sun is in the center of the solar system because the center of the Solar system itself isn't defined.

    Im sorry again.
     
  8. Feb 2, 2016 #7
    OK.So your question made me think about how an atom could look like. And voila...! I figured out "how" we know the nucleus is at the center of the atom. We have already obtained the electron probability distribution curves for various orbitals and all of them have a zero at the "center of the atom". In addition to that we also have a probable representation of a 2s orbital here.
    2sorbital.GIF

    There's a node right at the middle.

    In addition, if my question made sense and the nucleus was NOT at the middle, then I guess the 2s orbital would not be spherically symmetric. (Is this correct?)

    I guess I answered my own question. Thank you for the help...!

    By the way, we are not done with the Solar System part though. THAT, I do not agree with. I'm open for a debate...! :)
     
  9. Feb 2, 2016 #8

    ZapperZ

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    First of all, is there ANY other way, besides having a central-force system, to obtain the type of orbit exhibited by the planets? And not just orbits either. Note that this potential must also be able to describe all the different paths that we consider as "conic" sections.

    Secondly, if you look at what we have described for an atom, including all our ability to match the mathematics with our observation such as (i) the energy levels (ii) the geometry of the orbitals, etc... is there ANY other way to describe and obtain the SAME set of results without solving the Hamiltonian having a central nuclear potential?

    The problem that you have here is that you seem to think there is only ONE way to verify this. In physics, you must verify things QUANTITATIVELY. This means that there must be a mathematical model that makes a series, not just one, predictions, and then these predictions must MATCH a large set of experimental observations! And not only that, it is finally accepted when there isn't any other type of models that can make the same match with those experimental observations.

    So let's go back to the beginning. Do you have, or know of, a DIFFERENT mathematical model of a hydrogen atom, let's say, that does not have a central potential, and can produce ALL of the observed results that we know of from hydrogen atom?

    Zz.
     
  10. Feb 2, 2016 #9

    mfb

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    (Literally) millions of measurements. In every reasonable inertial system and for every reasonable definition of "center", the Sun is very close to the center.
    That is a more modern approach. Quantum mechanics didn't exist at the time the atomic nucleus was discovered.
     
  11. Feb 4, 2016 #10
    I never said that the force would be non-central. OK, so as we discuss this, are we considering the focii of an ellipse to be centers? If that is the case, then my arguments are obviously invalid.

    I do not know about the central nuclear potential. That is probably because I'm just 17 and I haven't REALLY studied Quantum Mechanics. I'm guessing this has something to do with the reference potential energy.

    Again, the central potential. But I get the gist of it. Nope, I do not know of any other theory.

    So basically you are telling me that scientists got these results, drew up a theory and no one found anything else convincing enough so we accepted it? Sounds good enough.

    So we didnt know till 1930- something that the nucleus was at the center? I should have been born then. People would have called me a phenomenal thinker. :P
     
  12. Feb 4, 2016 #11

    mfb

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    Of course we (well, "we" - scientists back then) knew it. We just didn't have that specific argument available yet.
     
  13. Feb 4, 2016 #12

    e.bar.goum

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    But not so much earlier than 1930. The Geiger and Marsden experiments that showed that a nucleus (as a concentrated central region of positive charge) exists were performed between 1908 and 1913.
     
  14. Feb 5, 2016 #13
    How did "we" know it then? Intuition?
     
  15. Feb 6, 2016 #14

    mfb

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    See the previous posts. There were no reasonable model that has it far away from the center, but perfectly valid models where it is the center.
     
  16. Feb 6, 2016 #15
    Hi CrazyNinja, simple. It's because the model works. Do you have a proposition "theoretical" may be that suggest its on random locations, that it can be verified with tests? If you have, keep it, and publish a paper so that it can be peered reviewed. Just to imagine, if you are right, all the efforts of our forefathers in chemistry, physics and engineering will be disregarded. Probably, you'll be famous by it.
     
    Last edited: Feb 6, 2016
  17. Feb 18, 2016 #16

    Henryk

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    Let me get back to the original question. How do we know the nucleus is at the centre?
    Never mind the theory. Let's us just see what would happened if the nucleus was located somewhere off-centre of the electron cloud
    One result would be that an atom would have a permanent dipole moment !! And would affect things like dielectric constant, etc. and could be also detected directly by passing atoms through a non-uniform electric field.
    As far as I know, no experimental evidence of a permanent dipole moment of atoms has ever been observed.
     
  18. Feb 19, 2016 #17
    Do we know? I can just for argument sake think of the quarks as existing outside the electron cloud and the gluons which contribute to more of the mass creating a web around the electron cloud where the center of mass is in the center but none of the contents are, except for the electron but that isn't the nucleus we are talking about...
     
  19. Feb 19, 2016 #18

    mfb

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    That doesn't give any model consistent with observations.
     
  20. Feb 19, 2016 #19
    The high energy proton collisions seem to indicate that particles emerge from the central point would be the best example of why not?
    In my mind its like the singularity in a BH which seems to have all the mass concentrated in a point in the center, I'm not trying to be difficult or argumentative I'm just asking why. I'm also curious if there are lower energy examples of why it doesn't work.
     
  21. Feb 19, 2016 #20

    mfb

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    Everything is an example. You cannot even make a model that you could disprove.

    The Rutherford experiment would give different results if the nucleus would be light.
    Gluons are bound in the nucleus, otherwise nucleons would not look the way they do.
    The valence quarks carry a large fraction of the total energy, as shown by deep inelastic scattering.
    A light nucleus would lead to a different reduced mass for the electron, shifting the energy levels, in particular those of the inner electrons relative to the outer ones, and the relative energy levels for different elements.
    Fine-structure and hyperfine-structure would look completely different with a different nucleus.
    ...
     
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