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Electron shells in atoms: WHY do they exist as they do?

  1. Aug 2, 2014 #1
    I have seen similar posts, but I haven't seen what seems to be a clear and direct answer.

    Why do only a certain number of electrons occupy each shell? Why are the shells arranged in certain distances from the nucleus? Why don't electrons just collapse into the nucleus or fly away?

    It seems there are lots of equations and theories that describe HOW electrons behave, predictions about WHERE they may be located, etc. But hard to find the WHY and causality behind these descriptive properties.

    Thank you! Please be patient with me, new to this forum and just an amateur fan of physics.
    Last edited: Aug 2, 2014
  2. jcsd
  3. Aug 2, 2014 #2


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    The answer boils down to the fact that there are a few simple rules that govern how subatomic particles interact. One of the most important in regards to electron orbitals is the pauli exclusion principle. When it comes right down to it, we don't know why the fundamental rules of nature exist. We only know that they do, and asking "why" a fundamental rule exists is typically a pointless question, as there is no answer until another theory is developed (which then has its own fundamental rules that you could ask "why" about).
  4. Aug 2, 2014 #3
    Hi Drakkith. Thank you. Yes, have read about the Pauli Exclusion Principle. Understand it holds up well, but still very unsatisfying as to WHY electrons behave this way.

    Hopefully I can continue to keep asking "pointless" questions on this forum if folks will be patient with me. =)

    Thanks again.
  5. Aug 2, 2014 #4


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    Some things, like the pauli exclusion principle, arise from more fundamental rules. The PEP is due to the way the wavefunctions have to work for certain classes of particles called fermions. Other things, such as why a particle can be described by a wavefunction, just comes down to, "that's just the way it is". Unsatisfying maybe, but the truth.
  6. Aug 2, 2014 #5
    Does atom has any shells?
    How this theorie appeared?
  7. Aug 2, 2014 #6


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    Shells refer to specific energy levels that electrons in atoms and molecules occupy. They are not physical shells.


    The idea of shells came with the develop of quantum physics in the early 1900's.
  8. Aug 2, 2014 #7


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    Also I wrote this somewhere else too...
    The shells don't give you a certain radius around the nucleus. They give you definite energies.
    The QM treatment of the atom, gives you the probability of finding an electron within some radius range. The definite radii appear only as the classically behaving mean values of the radius.
    The probability of an electron collapsing in a nucleus is rather small (it can happen and is called electron capture) but it generally happens to heavier atoms (proton-rich nuclides). The electron can as well fly away (ionization) if it gets enough energy to leave its bound state.
    What do you mean "why" and "casuality"? Everything works fine with casuality in QFT... The Schrodinger equation doesn't have to be casual, because it's not a relativistic equation (is not Lorentz covariant). The why itself is unanswered because it makes no sense (at least to me as you ask it)- it's how nature works and how the eigenvalue-eigenstate equations work.
  9. Aug 2, 2014 #8
    Certainly, but each time such exact seems strange.
    By the way, bit chemie:
    how to obtain 2g Fe2+?
  10. Aug 2, 2014 #9


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    I'm sorry, I can't understand your post very well.
  11. Aug 2, 2014 #10


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    Its related to a deep theorem of Quantum Field Theory called the spin statistics theorem:
    'Spin statistics theorem implies that half-integer spin particles are subject to the Pauli exclusion principle, while integer-spin particles are not. Only one fermion can occupy a given quantum state at any time, while the number of bosons that can occupy a quantum state is not restricted. The basic building blocks of matter such as protons, neutrons, and electrons are fermions. Particles such as the photon, which mediate forces between matter particles, are bosons.'

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