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Easy question: why don't electrons spin into the nucleus?

  1. Mar 19, 2003 #1
    ive read the answer so many times, i just can't think of the reason now...

    since the protons are positive and the electrons are negatively charged, why don't the electrons simply fall into the nucleus?
    Last edited: Mar 19, 2003
  2. jcsd
  3. Mar 19, 2003 #2

    Tom Mattson

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    Electrons bound in atoms are described by standing wavefunctions that occupy nonradiating states, as opposed to the classical model according to which an electron should radiate energy and spiral into the nucleus.

    Is that what you are looking for?
  4. Mar 19, 2003 #3
    yeah thanks

    but, how would the electron "radiating energy" cause it to spiral into the nucleus as opposed to the other option---"nonradiating states"?

    in other words, can you briefly explain each of these 2 cases. thanks
  5. Mar 19, 2003 #4
    Classically you would have electrons orbitting the nucleus in ellipses -- just like planets and the sun. Except accelerating/orbitting charges radiate light, losing energy, and would spiral into the nucleus. That what you wanted, or was it why an accelerating charge radiates?
  6. Mar 20, 2003 #5
    If there is a proton around, it will attract an electron and cause it to accelerate, so how does an electron occupy a non-radiating state inside an atom?
  7. Mar 20, 2003 #6


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    Greetings !
    Like Tom said - "Electrons bound in atoms are
    described by standing wavefunctions that occupy
    nonradiating states ".
    These "states" are called Orbitals. Orbitals, unlike
    orbits, are symmetric clouds passing through the
    nucleus (with their sharper edges - like water drops
    connected). The Pauli Exclusion Principle is the result
    of QM calculations and it determines that no more
    than two electrons can occupy every orbital.

    "Does dice play God ?"

    Live long and prosper.
  8. Mar 20, 2003 #7

    Ok, let's take the hydrogen atom for example. It has one electron and one proton. If protons and electrons attract, what keeps the electron away from the proton?
  9. Mar 20, 2003 #8


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    Quantised energy levels for the orbiting electron.
  10. Mar 20, 2003 #9
    Frienly tutorial

    Hi Drag,
    FYI: You have it a little backward because the
    QM calculations are the result of the Pauli principle and
    that principle was the result of the classic discovery, by dint
    of the characteristic x-ray spectrum, that showed, experimentally,
    the significant difference between k-a and k-b radiations.
    Furthermore, the Pauli principle does not merely limit an orbital to two electrons but demands that they must differ in some trait other than the similarity of their charges. QM may very well assume that inertial spin – up vs. down – is a sufficient difference; however. because charge and mass are intrinsically coupled and further, because dipolar spin is markedly feebler than dipolar magnetism, the classic modeling prefers the strong attractiveness of the latter.

    It is apparent to me that your reference to “clouds passing through the nucleus” suggests that drawings found in chemistry texts, that look like balloons and dumbbells, have been mistakenly interpreted by you as representing some kind of orbital property. In reality, those drawings represent the probability of the position/momentum paradox associated with single (usually orbitally uncoupled valence) electrons that are used to demonstrate concerning the uncertainty indicated by Heisenberg. [E.g. the nitrogen atom has three valence electrons and a drawing of that atom would show three independent orthogonally disposed dumbbells. When ammonia gas is formed, by three covalent bonding quantum orbitals, the so-called hydrogen bonds remain orthogonally disposed.]

    Your audience is appreciated. Thanks, Only The Messenger
  11. Mar 20, 2003 #10
    I think what you are looking for is this:

    the minimum average distance between an electron and the proton is limited by heisenberg's uncertainity principle.the electron must have a zero point energy for consistency of HUP. this will be violated in case electron falls into the nucleus.
    this is analogous to the 'fermi pressure' in case of neutron stars not collapsing under gravitation.
  12. Mar 20, 2003 #11

    Tom Mattson

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    I think the problem is in the sentence above.

    Electrons do not have well-defined trajectories, such as those we are accustomed to in everyday life. Thus, we do not speak of a position function x(t) for the electron. Since acceleration is just the second time derivative x''(t) of position, we do not speak of that for quantum particles, either.
  13. Mar 20, 2003 #12

    the HUP places a limit on measurement of the states of the proton and the electron but I don't see how that principle can force the electrons from falling into the proton nucleus of a hydrogen. Isn't that like saying because I am blind, the electron won't be attracted by the proton?

    Is there some other stuff in the hydrogen atom besides the electron and proton?
  14. Mar 20, 2003 #13

    Tom Mattson

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    The HUP does not acquire its physical importance from the process of measurement, so your blindness has nothing to do with it. HUP bears on the problem because it is a consequence of the fact that the electron is a standing wave inside the atom. Standing waves can only exist at discretely defined energies, and in the case of atoms, E=0 is not one of them.
  15. Mar 20, 2003 #14

    thx, that makes more sense. Is the electron like stretched out into a closed loop inside an atom and the loop is oscillating?
  16. Mar 20, 2003 #15
    The electron is described by a probability function...it's a function that depends on space...and it's value is a complex number (a+ib)...the integral of the "modulus"(translation) of this function over the entire space must be 1;
    the value of the function is the amplitude of probability;
    the "modulus" (sqrt(a^2+b^2)) is the probability that the particle is located at that "x"...so...you see...there's no "physical" wave...like an oscillator...
  17. Mar 20, 2003 #16

    now I'm confused again. Someone just said that the electron is actually a standing wave inside the atom and that is why they don't fall into the nucleus. It seems you are now arguing again it is the probability function, i.e. a consequence of the HUP that prevents the electron from falling into the nucleus.

    A hydrogen atom only has a proton and an electron- if opposite charges simply attract each other, what keeps them apart besides a probability function?
  18. Mar 20, 2003 #17
    Welcome to the world of quantum mechanics! :wink:

    First, do you get the classical picture of how the electron doesn't fall into the nucleus? Same reason the Earth doesn't fall into the sun: it's orbitting around it.

    In QM, it's sort of similar. Except an electron isn't really a particle, but a wavefunction, which is like a probability wave -- a standing wave if it's bound in an atom. This has a minimum energy, which gives the 'closest' orbit it can be in around the nucleus. But, when you look for the electron, you always find it in one place, with its probability given by the amplitude of its wavefunction (squared).
  19. Mar 20, 2003 #18
    Umm ... not the same.
    The electron is electrically charged, the moon is not.

    Sometimes you find the answers where you don't expect them.
    See this PF topic
  20. Mar 20, 2003 #19

    Tom Mattson

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    It is close enough.

    The electron is electrically charged, and it the electric force that keeps it in orbit.

    The moon is "gravitationally charged" (IOW, it has mass), and it is the force of gravity tht keeps it in orbit.
  21. Mar 20, 2003 #20
    I know Tom, this is not what i meant.
    What i meant is that an moving electrically charged body will produce an electromagnetic wave (well, at least this is what they thought to be always true in the old days), while a moving gravitationally charged body will not produce any kind of wave that may take energy from it (at least not till the thoeries of today, and not till we find clues of Gravitons).
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