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    So i have just started looking at my chemistry AS book and am going over AS electron configurations.

    Ive seen that Chromium has a atomic number of 24.
    So i guessed the electron configuration would be:
    1s^2 2s^2 2p^6 3s^2 3p^6 3d^6

    But the actual one is:
    [Ar] 3d^5 4s^1

    Where am i going wrong??

    Thanks
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    (Original post by TSR778899)
    So i have just started looking at my chemistry AS book and am going over AS electron configurations.

    Ive seen that Chromium has a atomic number of 24.
    So i guessed the electron configuration would be:
    1s^2 2s^2 2p^6 3s^2 3p^6 3d^6

    But the actual one is:
    [Ar] 3d^5 4s^1

    Where am i going wrong??

    Thanks
    There are some elements that have exceptions to how you usually fill up electron configurations in order. Chromium is one of them that you need to know.. Copper may be another but I am rusty (pun intended) on chem knowledge. You need to be able to explain why the 4s orbital has one electron in it, rather than 3d6. This is because (seek the proper explanation out from the textbook or your teacher) in 3d6, you would go from 3d5 to 3d6. In 3d5, each pair of two has one electron in it (difficult to do without a diagram but it's 1 1 1 1 1, if you like (5 pairs of 2 make up 10 for 3d10). So when you 'go' to 3d6, one electron would be added to give 2 1 1 1 1 as the five pairs in the d orbitals. The '1' that it gets added to doesn't like this and so there's repulsion going on, and it's 'more stable' (or has a lower energy level.. I think) for the '1' electron to go to 4s1 instead of 3d6.
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    (Original post by SeanFM)
    There are some elements that have exceptions to how you usually fill up electron configurations in order. Chromium is one of them that you need to know.. Copper may be another but I am rusty (pun intended) on chem knowledge. You need to be able to explain why the 4s orbital has one electron in it, rather than 3d6. This is because (seek the proper explanation out from the textbook or your teacher) in 3d6, you would go from 3d5 to 3d6. In 3d5, each pair of two has one electron in it (difficult to do without a diagram but it's 1 1 1 1 1, if you like (5 pairs of 2 make up 10 for 3d10). So when you 'go' to 3d6, one electron would be added to give 2 1 1 1 1 as the five pairs in the d orbitals. The '1' that it gets added to doesn't like this and so there's repulsion going on, and it's 'more stable' (or has a lower energy level.. I think) for the '1' electron to go to 4s1 instead of 3d6.
    Thanks a lot, i kind of understand now
 
 
 
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