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Professor Lavelle mentioned writing the d orbital before the s orbital when doing electron configurations, and he used chromium as an example so it was 3d4s. For elements further down in the periodic table like molybdenum which is right below it, it would end in 4d5s but do we still write it as 3d4s even if it's in the middle?
The one he was talking about was a specific exception regarding chromium. I believe for certain elements, you would write 4s before 3d because 4s has a slightly lower energy, but for others, it would be normal and you would write 4s after 3d.
The reasoning behind the order is that you're supposed to write it in order of lowest energy to highest energy. Because shells that are further away from the nucleus have less electrostatic attraction, further shells have less energy (double-check me on this?).
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