3d Subshell
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3d Subshell
Can someone elaborate on whether the 3d orbital is normally higher or lower than the 4s orbital? I'm not really understanding the exceptions and why these exceptions exist...Like why is the electron configuration for chromium [Ar](3d^5)(4s^1) and copper [Ar](3d^10)(4s^1)?
Re: 3d Subshell
The 3d orbital is lower energy after element 20 (Sc) because of electron shielding. Electrons are added the 4s shell first because it is more stable, but when electrons are added to the 3d orbital, shielding becomes a factor. The 4s shell is higher energy and has less influence from the nucleus, so it is the valence shell. The same sort of logic applies to the 3d5 and 3d10 elements, 3d104s1 is more stable than 3d94s2 so the electrons arrange themselves in that order first.
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Re: 3d Subshell
You just need to memorize the exceptions but for why they exist its related to how stable the atom would be. It would be more stable for chromium to have a half filled subshell of d and take it from s than to have a full subshell of s and 3d4 and in concept its the same for copper as well.
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Re: 3d Subshell
The 4s shell is still a higher energy that 3d orbits. The reason that electrons are assigned to 4s first is because it is a much more stable and symmetrical shell. When we look at the exceptions to the rule, Chromium and Copper, we also want to apply this "symmetrical thinking". Even though 4s is more stable, Chromium and Copper having electrons in every suborbit of 3d leads to a stable, "more symmetrical" ground state for the atom.
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Re: 3d Subshell
Before you hit scandium, the 4s subshell has a lower energy than the 3d and is filled first, as seen in potassium and calcium. But from scandium onwards, the 3d subshell has a lower energy and is filled first. The reason why there are electrons assigned to the 4s subshell even though the 3d is not filled up is because it leads to less electron repulsion and greater stability. But with chromium, the exception occurs because have one electron in each of the five orbitals leads to the greatest stability, and with copper, having the entire 3d subshell filled up also leads to the greatest stability.
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