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    In terms of GCSE, outer shell has 2 steps of being full. So it's full at 8 but for potassium and calcium, you have 1 and 2 extra electrons in that shell respectively. Basically it's actually an extremely simplified and inaccurate model that is good at explaining bonding between small atoms.
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    (Original post by aspen fernandes)
    Why does argon a nobel gas have 18 electrons in total so 8 in the 3rd shell when it should be 18 electrons
    Saying Nobel gasses have a full outer shell is an oversimplification. What should be said is that nobel gasses have a full set of s and p electrons in their outermost shell. A shell can hold max 2 s electrons and 6 p electrons, so total 8 electrons in the outer shell of every nobel gas, even when the outer shell can hold more than that. All the outer electrons are paired, making the elements reluctant to bond still, so it's like they've got a full shell.
 
 
 
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