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We all know that potassium, the 19th element on the periodic table has the electronic configuration of 2.8.8.1. However, why not 2.8.9? In the element scandium, it has the electronic configuration of 2.8.9.2. I would like to know why potassium is not 2.8.9.

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You could actually have the 2.8.9 configuration, putting the last electron in a 3d rather than 4s orbital. But the 2.8.8.1 one is lower in energy and thus more stable. How electrons fill subshells in neutral atoms to make the lowest energy configuration can be seen here.

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