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Stability of Atoms with half-filled orbitals

  1. Nov 5, 2012 #1
    I have observed that when the outermost orbital is half filled (like Nitrogen 1s2 2s2 2p3), the atom has a higher stability (or a lower binding energy). Why is this so? I have heard it has to do with the fact that electron spin is maximized at that point but it still does not make sense to me.

    Note: Please assume that I have very less knowledge of quantum mechanics. However, I am familiar with electron spin and quantum numbers, the Aufbau rule and the Pauli Exclusion Principle.
  2. jcsd
  3. Nov 7, 2012 #2

    I'm an Ametuer, but I believe the real reason for the stability of half filled shells is due to a decreased likelyhood of a reaction to occur that will fill or empty the 2p3 electron shell. Please take what I say with a grain of salt, because I'm more of a math and physics guy, but Chemistry is related to that in alot of ways. Basically, half full outer shells for Nitrogen and other atoms appear to be more stable because they are in fact less stable. When you add one more electron, you raise the Coulomb energy of that orbital, so it has an uneven number of opposite spins, and that sort of throws it out of balance and the orbital tends to lose the electron because it requires less energy if the spins are even.

    I'm sorry if I'm not very good at explaining this, but basically a reaction has to occur that donates alot of electrons or takes alot away in order to be favored over maintaining a balance because of the level of instability of shells that are slightly more than half full or slightly less than half full.

    Hopefully someone will come along and either correct me or explain what I'm saying a little better.

    Thanks, and good luck!

    -Justin Hall
  4. Nov 20, 2012 #3


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    Staff: Mentor

    Please elaborate. Atom per se has no binding energy, unless it is a part of a molecule,
  5. Nov 21, 2012 #4
    I was asking about the electron's binding energy to the nucleus.
  6. Nov 23, 2012 #5


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    When filling up a shell (or sub-shell) in an atom, the electrons in that shell, being all in orbitals of approximately the same size, are quite bad in shielding each other from the charge of the nucleus. Hence these electrons will be increasingly stronger attracted to the nucleus the more the shell is filled up due to the increase of effective nuclear charge.
    This has been formalized in the Slater rules: http://en.wikipedia.org/wiki/Slater's_rules
    However when you attemt to add a further electron to a half filled sub-shell, necessarily two electrons have to occupy the same orbital with increased Coulomb repulsion. So half filled and full shells are kind of optimal.
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