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Electron Configuration

  1. Nov 11, 2007 #1
    [SOLVED] Electron Configuration

    Something in chemistry really has me perplexed. We are doing electron configuration stuff. I think I have a fairly good grip on it after I was able to let go of the Bohr Model.

    My question is this. Chromium has 24 electrons which should fit into the first 3 shells 3s, 3p, 3d yet it uses the 4th shell. Why does it not fill up the third shell?

    The way it seems it should be to me is 1s2,2s2,2p6,3s2,3p6,3d6 not 3d5,4s1

  2. jcsd
  3. Nov 11, 2007 #2
    if i remember, the atom becomes more stable when a subshell is either fully filled/half filled. so with the 3d6 loses one electron to make it half full and places it in the 4s subshell to make that half filled as well making the atom more stable.
  4. Nov 11, 2007 #3
    It does this to reduce electron-electron repulsion (making the atom more stable). There's a whole list of atoms with anomalous configurations, off the top of my head I can think of Cr, Cu, Nb, Mo, Ru, Rh, Ag, La, Ce, Pt, Au, Ac, Th, Pd. There are two or three more.
  5. Nov 12, 2007 #4
    In the case of Cr, if it loses the one elctron to the 4s shell, that make 4s half full, but 3d is not half empty. At any rate, I think I see what you are saying.
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