4s and 3d orbitals

[Jocelynn Franco 1L]

How do we know the exceptions for energy levels?
In my notes, I wrote down that once the 4s orbital is filled and electrons enter the 3d-orbital, then the 3d-orbital is lower in energy than 4s.
How does this work and why does this make sense?

[grace salvestrin 1J]

Hi Jocelynn!

This happens because the 3d level cannot be reached until after the 4s level has been filled, which is because of their order on the periodic table. Once 3d is reached, the level 3 has lower energy than 4 so the way their order is written has to be rearranged.
I hope this helps some!

[Jiayin Yola Yan 1G]

I believe that you always write 3d before 3s when 3s is not filled. But when 3s is filled, then write 4s first

[Kiku Shirakata 2A]

Hello,

This concept confused me too. We learned that 4s orbital is slightly less in energy than 3d orbital initially, but once 4s is filled and starts filling up 3d orbital, 4s orbital is now higher in energy. As more and more shells get added and the further the shells go, energy gap between shells get smaller and smaller. As these energy differences get smaller, some things shifts and exceptions occur.

Hope this helps!

[Jessica Sun 2I]

As stated, when both 4s and 3d are unfilled then 4s has a slightly lower amount of energy than 3d. Once 4s is filled, however, it becomes slightly higher in energy than 3d.

[805752034]

When both 4s and 3d orbitals are not filled up, it shows that 4s has a little bit less energy than 3d. This is why you fill up 4s orbital first.

[Tyler Olson 1E]

Initially there is a lower amount of energy in the 4s sub level compared to 3d before they are filled. After they are filled, the 4s level ends up having more energy so that is why it is filled first, because the initial amount of energy is lower.

[tashyavakil 2K]

4s fills first however as soon as 3d has at least one electron, it is lower in energy and is written before 4s