Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Odd electron configuraitons

  1. Aug 24, 2013 #1
    What causes that some elements have an electron configuration that is not in agreement with Hund's rule (Cu, Cr, Ag, Au...)? E.g. why does chromine have a valent electron configuration of 4s1 3d5 instead of 4s2 3d4. The former configuration is said to be more energetically favored, but why?
     
  2. jcsd
  3. Aug 24, 2013 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    The simplest answer here is that if you look at the two orbitals (4s and 3d), the 3d, on average, is further away from the nucleus than then 4s. Thus, the 3d is shielded more from the nucleus, and thus, will have a higher energy than the 4s orbital.

    Zz.
     
  4. Aug 24, 2013 #3
    3d has higher energy, but why is it filled before 4s becomes completely filled?

    I don't need the simplest answer, I need the answer that is nearest to the truth. I'd like a quantum mechanical explanation.
     
  5. Aug 24, 2013 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    But that's the point, 3d does NOT have a higher energy! If it does, it won't get filled up ahead of the 4s! It is closer to the nucleus than the 4s on average!

    That is the QM explanation. If you don't care for it, solve the Schrodinger equation.

    Zz.
     
  6. Aug 24, 2013 #5
    In vanadium, 4s is filled before 3d(!), what causes the difference between Mn and Cr orbital filling?
     
  7. Aug 24, 2013 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Yeah? Isn't that what we have been discussing all along? I feel like I keep having to repeat the same thing.

    I'm confused. What is it exactly that you are asking? Mn and Cr have different number of electrons. Why should they have the same "orbital filling"?

    Zz.
     
  8. Aug 24, 2013 #7
    Okay, I will reformulate my quesiton:
    Vanadium has a completely filled 4s orbital and 3 electrons in the 3d orbital, by analogy, chromine should be 4s2 3d4, but instead it is 4s1 3d5. Why isn't the 4s orbital in chromine filled completely?
     
  9. Aug 24, 2013 #8

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Obviously, it is a lower energy to half-filled the 3d orbital than to fully fill the 4s orbital.

    At some point, we simply can no longer simply try to give a hand-waving explanation on why this occurs, because the physics of multi-electron system gets rather complex. Anything beyond Helium will require sophisticated methods, such as DFT, to get the solution to such atoms.

    Zz.
     
  10. Aug 24, 2013 #9
    "At some point, we simply can no longer simply try to give a hand-waving explanation on why this occurs, because the physics of multi-electron system gets rather complex. Anything beyond Helium will require sophisticated methods, such as DFT, to get the solution to such atoms."

    That's what I was asking for. Thank you for the answer.
     
  11. Aug 25, 2013 #10

    DrDu

    User Avatar
    Science Advisor

    Maybe a simple explanation is that advancing along a period, the effective nuclear charge seen by the valence electrons increases as the valence electrons are not very effective in shielding each other from the nuclear charge. Compare the Slater rules Slater's rules - Wikipedia, the free encyclopedia.
    Now with increasing (effective) nuclear charge, e.g. in going from V to Cr, the effects of many electron effects in an atomic problem become less pronounced and the ordering of the orbitals approaches that found in a hydrogen atom, i.e. 3d falls below 4s.
    This is formalized using "Z expansion".
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Odd electron configuraitons
  1. Electrons and protons (Replies: 12)

Loading...