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So I know in lecture we have been discussing how the d orbital is written before the s. I have never understood why this is. Could someone explain why this is and does it happen every time no matter if either orbital is full? Is there ever a time when the d orbital is written after the s in the ground state figs?
The 4s orbital is only lower in energy before the 3d orbitals have any electrons, this is why you fill the 4s first. However, after you fill the 4s and then add one or greater electrons to the 3d, the 4s becomes the higher energy orbital due to shielding from the electrons in the 3d. So, generally, 4s < 3d in energy before the 3d has electrons, but the 4s > 3d after the 3d receives an electron(s). This is why you fill the 4s first, but also ionize it first, because its relative energy changes depending on the presence or absence of electrons.
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