4s becomes higher energy

[Isabelle Fontanilla 1I]

While I know that electron shells fill up based on the Aufbau principle(lower energy levels fill up before higher energy ones), I do not understand how the 4s becomes higher energy after it fills up and 3d becomes lower energy. I also don't understand why elements before atomic number 21 fill in the 4s orbital before the 3d orbital. I guess my real question is: "How was the Aufbau principle derived?"

[MichaelMoreno2G]

I also had a similar question regarding this, if someone can please answer I would greatly appreciate you.

[armintaheri]

The 4s subshell is lower in energy than the 3d subshell, so it fills up first. The 4s becomes higher in energy after filling up, so if you ionize the atom, electrons are lost from the 4s subshell before the 3d subshell. But that's not how I learned it so it just confuses me to think of it that way. The way I learned it, subshells are more "stable" or lower in energy when they are either full or half-full. So let's say you want to remove an electron from zinc, which has a full 4s and a full 3d subshell. If you remove it from 3d, you end up with a full 4s subshell, which is good and stable, but a 3d shell that is 9/10 full, which is not stable. On the other hand, if you take that electron from the 4s shell, you end up with a 4s subshell that is half-full and therefore stable, and a 3d subshell that is completely full and also stable. That's why when you ionize you take electrons from the 4s before the 3d. That's the only way both subshells can be stable. That explanation makes more sense to me than saying the 4s increases in energy after filling up.

The full/half-full rule also explains why copper fills 3d before filling 4s. Full 3d and half-full 4s is more stable than full 4s and 9/10 full 3d.

I think part of the confusion is that 4s has a 4 in front of it, so it sounds like it should be higher energy than 3d, but the energy level of orbitals depend on their principal quantum numbers ( the 3 and the 4) and the kind of orbital they are (the s and d). So just because 4s has a 4 doesn't mean it should be higher in energy than 3d, since they are different kinds of orbitals, so you can't directly compare them.

[Nawaphan Watanasirisuk 3B]

The 4s subshell is lower in energy than the 3d subshell, so it fills up first. The 4s becomes higher in energy after filling up, so if you ionize the atom, electrons are lost from the 4s subshell before the 3d subshell. But that's not how I learned it so it just confuses me to think of it that way. The way I learned it, subshells are more "stable" or lower in energy when they are either full or half-full. So let's say you want to remove an electron from zinc, which has a full 4s and a full 3d subshell. If you remove it from 3d, you end up with a full 4s subshell, which is good and stable, but a 3d shell that is 9/10 full, which is not stable. On the other hand, if you take that electron from the 4s shell, you end up with a 4s subshell that is half-full and therefore stable, and a 3d subshell that is completely full and also stable. That's why when you ionize you take electrons from the 4s before the 3d. That's the only way both subshells can be stable. That explanation makes more sense to me than saying the 4s increases in energy after filling up.

The full/half-full rule also explains why copper fills 3d before filling 4s. Full 3d and half-full 4s is more stable than full 4s and 9/10 full 3d.

I think part of the confusion is that 4s has a 4 in front of it, so it sounds like it should be higher energy than 3d, but the energy level of orbitals depend on their principal quantum numbers ( the 3 and the 4) and the kind of orbital they are (the s and d). So just because 4s has a 4 doesn't mean it should be higher in energy than 3d, since they are different kinds of orbitals, so you can't directly compare them.

Thank you so much that cleared a lot of things up for me!