d vs s orbitals

[Joey_Okumura_1E]

In atoms that contain both 3d and 4s subshells...
1. Are electrons are added to the 4s subshell before they are added to the 3d subshell?
2. If a cation were being formed, would the electron be taken from the 4s orbital?

If the answer to both questions is yes, don't they contradict each other? I thought 4s orbitals were of higher energy than 3d orbitals which would explain why electrons would be removed from the 4s subshell to create a cation. However, I also thought that electrons were added to the lowest energy shell first.

3. Does this same trend apply to 5s and 4d, 6s and 5d, etc.?

[Isabel_Eslabon_2G]

Hello. From how I understand it:

1. Electrons are added to the 4s orbitals before the 3d orbitals because the 4s orbitals are lower in energy when both are unoccupied. The 4s and 3d orbitals are very close in energy. After 4s orbitals are occupied, electrons must enter 3d state (then 3d is lower in energy than 4s).

2. To form cations when an atom has electrons in the 4s and 3d orbitals, you need to remove electrons from the 4s orbital first because the occupied 3d orbitals are lower in energy than 4s (there are exceptions to removing electrons, such as Cr and Cu, though).

3. I am pretty sure this applies to 5s & 4d, and onwards.

Basically, if the 4s and 3d orbitals have no electrons, then the energy of 4s is less than 3d. But if the 4s orbital has electrons while the 3d doesn't, the energy of 4s is greater than 3d.
Looking at my notes, this information comes from the lecture on 10/26 (Lavelle explains it better there).

[Gillian Gabrielsen2F]

For s block elements, like K and Ca, 4s fills up first. To my understanding this is because 4s and 3d are close in energy level and 3d is more compact, so the electrons are held closer together which causes more repulsion, so filling up 4s first is more efficient energy wise. However, for transition metals (d block), 3d would fill up first, and then the two other electrons end up in 4s, since there's less repulsion and takes less energy compared to staying in 3d. This also applies to 4d and 5s, etc
To form cations, 4s electrons are lost first since they're the farthest from the nucleus so the nucleus has a weaker pull on them. 4s is part of the 4th shell while 3d is part of the 3rd shell.

If you want a better, more in depth explanation, I found these sites helpful. Essentially, Aufbau is a way to figure out electron configurations correctly, but not necessary the order they're being filled. https://chem.libretexts.org/Bookshelves ... _Chemistry)/Electronic_Structure_of_Atoms_and_Molecules/Electronic_Configurations/The_Order_of_Filling_3d_and_4s_Orbitals and http://ericscerri.blogspot.com/2012/06/ ... -find.html

[Jasraj Parmar 3H]

Yes, 4s fills up before 3d. I was a little confused at first also. The trend also continues for 5s,4d and 6s,5d.

[Jacquelyn Challis 2B]

Just to clarify, when writing electron configurations, would you write 3d or 4s first? I've seen it done both ways and I'm getting confused.

[Racquel Fox 2I]

Just to clarify, when writing electron configurations, would you write 3d or 4s first? I've seen it done both ways and I'm getting confused.

In order to fill them in the right order, you can write 4s first, but proper electron configuration is to put it back to 3d, 4s.

[Jacquelyn Challis 2B]

Got it, thanks!!

[Chinmayi Mutyala 3H]

They don't contradict each other because s orbitals have lower energy, so that's why electrons are added to 4s before 3d and since they have lower energy, they are easy to remove so when a cation is formed, they are removed from 4s orbitals.