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    For Chromium
    Cr3+:1s2 2s2 2p6 3s2 3p6 4s0 3d3
    Why is 4s^0???? and why does it then go to 3d^3?
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    If you want a really good explanation I'd recommend going onto chem guide, it's better than anything I could do.
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    These are the notes I made on it, hopefully this will be able to answer your question?
    (Original post by SAB10)
    For Chromium
    Cr3 :1s2 2s2 2p6 3s2 3p6 4s0 3d3
    Why is 4s^0???? and why does it then go to 3d^3?
    An orbital; bit of space that an e- moves in, orbitals in the same subshell have the same energy. e-
    in each orbital
    have to ‘spin; in opp. directions, spin pairing.
    Chromium:
    To achieve a more stable arrangement of lower
    energy, one of the 4s e- promoted into 3d e-
    to give
    6 unpaired e- with lower repulsion (would be 4 else)
    Copper:
    One of the 4s e- promoted into the 3d orbital
    In an ion:
    4s e- are ionised before 3d as they are in a lower energy state when the 3d orbital has e-
    in it. This is because the 4s
    orbital is further away from the nucleus, so when 3d orbitals present, which is closer to the nucleus, it shields the
    electrons in 4s.
    The 3d orbitals have the lowest energy, but as we add electrons, repulsion within the orbital can push some of
    them out into the higher energy 4s level.
    http://www.chemguide.co.uk/atoms/pro...4sproblem.html
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