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Protons and electrons

  1. Apr 18, 2004 #1
    If opposites attract, why dont electrons crash into the nucleus. Ive been told its because the electon is spinning so fast, it orbits, but I know this cant be so because then at absolute zero, the electron should then crash inward or even at a higher temperature. Or is it the "weak nuclear" forces? Please help!
  2. jcsd
  3. Apr 18, 2004 #2
    Yup. It's the strong force i believe that holds atoms together. Either that or the weak force. But it's one of the two, because the electromagnetic forces are huge and must be fought by something!
  4. Apr 18, 2004 #3
    Actually, this is more of a quantum effect. See The Bohr Model
  5. Apr 18, 2004 #4


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    There are many explainations as to why the electron does not fall into the nucleus, the one you mentioned being a classical explaination. The quantum explaination is to do with the wave nature of the electron and that is that it can't get closer to the nucleus because it occupies the lowest energy level that corresponds to how many wavelengths will fit around the atomic nucleus. I find this the easiest way to picture why an electron will not go beyond a certain level (except of course with extreme outside forces).
  6. Apr 18, 2004 #5
    ok, so is it nuclear forces, quantum effect, or wavelegnth properties, or electron spin? Is there a definite answer? :confused:
  7. Apr 18, 2004 #6


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    The only that makes sense to me is the energy of the wavelength.

    If it loses any energy, for some reason, there is a good possibility that it will hit the nucleus or proton.

    If it gains energy, like getting hit by Gamma or X-Rays, it will gain energy and escape the atom.

    I would say wavelength's is the definite answer.
  8. Apr 18, 2004 #7
    cowboy_mortician asked: "ok, so is it nuclear forces, quantum effect, or wavelegnth properties, or electron spin? Is there a definite answer?"

    "Quantum effect" and "wavelength properties" are both references to the uncertainty principle, the underlying law of nature that explains the electron's stability at a non-zero distance from the nucleus.

    Nuclear forces and electron spin have nothing to do with it.
  9. Apr 19, 2004 #8


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    As you have seen via all the responses to your question, this isn't a trivial thing to answer, at least it isn't trivial to answer if you want a simple picture or analogy. One can try explaining it by saying that an electron is a "wave" and that the smallest standing-wave configuration is the fundamental wavelength, very much like the Bohr-Sommerfeld quantization model. But this would be inaccurate also.

    The second problem on why this is difficult to answer is because we have this picture of a well-defined "electron" zipping around in an atom, very much like a planetary model, etc. That perception in itself can be the root of the difficulty in trying to explain and answer this problem. When you ask why doesn't the electron "crash" or get arbitrary close to the nucleus, I could easily reply "But it does, but not in the way you think!" [I will explain this later]

    H atom is one of the MOST well-known system in physics. In QM, it is one of the few exactly-solvable system (He atom is the other - the rest of the elements have not been solved exactly). When we ask how the H atom behaves, especially in terms of the dynamics of the electron within this atom, we have to first of all solve the Hamiltonian or the Schrodinger equation of the system. For most physics majors, this can be a painful exercise, because it is almost a requirement that one MUST know how to do this. When you solve this, you will find that the LOWEST possible state satisfying the Schrodinger equation with the appropriate H atom boundary conditions contains three quantum numbers n,l,m with values n=1, l=m=0. This state corresponds to the LOWEST energy possible given the H-atom configuration. There's nothing lower.

    Now this derivation sort of answers your question on "why", i.e. it is because it is the lowest state possible with the given configuration and so, there's no other places for the electron occupy to get any closer. But unfortunately, it doesn't completely answers why. It merely says that based on QM, I can give you a description of the H atom that accurately match ALL of the experimental observations. QM doesn't allow us to go beyond that because such info are not imbedded within the QM description (or is this just a objectivist or solipsist view?).

    However, the interesting thing here is that, knowing the solution for the lowest state, where is the electron? This would explain what I said earlier that the electron in the H atom does actually get very close to the nucleus, but not in ways you think. The "radial solution" of the H-atom, i.e. the part that describe the electron's position from the center of the atom, has a very interesting profile for the lowest state. The expectation value for |nlm>=|100> of the radial function actualy PEAKS at the origin,i.e. where the nucleus is. So the radial wavefunction seems to indicate that the highest probability of finding the electron is actually right at the nucleus! [Note: rR_nl is zero here].

    The major caveat here that one needs to keep in mind is that this is based on a strictly non-classical view of what an electron is. The wavefuction does not describe a "well-defined" particle known as an electron, but rather it is describing the PROPERTIES of the electron that seem to be "smeared" in the space of the atom. It only based on this that we can accurately explain all the phonomena associated with the H atom.

    If you wish to see the painful detail of such a derivation, you're welcome to look at the link below:


    It will give you an idea why I mentioned in the beginning that answering such a question isn't as trivial as it appears.

  10. Apr 19, 2004 #9


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    At the terrible risk of confusing cowboy_mortician, there is an example of an orbital electron 'crashing into' the nucleus - electron capture, a mode of radioactive decay in which a proton 'captures' an orbital electron (usually in the K shell) to become a neutron (and emits an anti-neutrino).

    In terms of the excellent description Zapper Z provided, can you see how this 'capture' might happen? (It's surely a nonsense in the 'solar system' view of an atom :wink: )
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