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HELP! - Subshells watch

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    I need some help and clarification on subshells as it is in my uni syllabus, but have never done chemistry before apart from in GCSE's.

    I'm terrible at trying to expalin things so bare with me

    I understand the S, P, D and F subshells and how many subshells it has and how many electrons each on carries, but I was told that sometimes one subshell is missed so that the next one could fill completely.

    So I was trying to understand why this happens and when it happens?

    e.g Ca (20) is - 1s2 2s2 2p6 3s2 3p6 4s2

    Why does it miss the 3d subshell and skip straight to 4s2?
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    It's all about relative energy, the 4s orbital is lower in energy than the 3d. It's much like water flowing down the hill, it chooses the route of least resistance - the 'least energy path'.

    Look at the periodic table - do you know they're split into s, p, d and f blocks? (sorry, I'm not sure how much you know)

    If you follow the atomic number order, you'll see which shells fill first in general (there are exceptions).
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    In short, electrons fill the subshell with the lowest energy and for some reason 3d subshells have MORE energy than 4s subshells so 4s are filled before 3d...it gets a little bit tricky at Chromium and Copper because these elements give up one electron from their 4s shell so each electron in 3d are paired up/one unit is filled - 'half shell stability'


    (AS understanding, sorry for not being clear)
 
 
 
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