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So a few people have already asked why 4s fills up before 3d, which I understand. What I wanted to know is why is it that the 4s orbital is lower in energy than the 3d orbital before it is filled, but is higher in energy after it is filled? Someone may have asked this already but I couldn't seem to find a thread for it. If there is an existing thread could someone please send me the link.
Once an electron is in the 3d sub shell, it is lower in energy because the 3d electrons are in the n=3 shell and 4s electrons are in the n=4 shell. As a result, the 3d electrons are listed before the 4s electrons when writing out configurations.
So to my understanding, 4s is lower in energy than 3d for s-block elements because the s orbital shape is a sphere and is "simpler" or lower in energy than the complicated energetic shape of the p orbital. 4s electrons are removed first though, because they are farther from the nucleus than 3d electrons (4>3).
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