Ordering of s and d orbitals

[TheresaDsilva4A]

In my AP Chem class, I learned to write the 4s subshell before the 3d subshell when writing an electron configuration. However, during the lecture and in the examples/problems from the textbook, the 3d orbital is written prior to the 4s orbital. (I also remember Dr. Lavelle saying that this is the proper way to do so.) Can someone please explain to me why this is the case? I had always assumed that because the 3d subshell is filled after the 4s subshell it should be written after it. Thank you so much!

[KHowe_1D]

I was taught the same thing in high school but I think that it was because it was easier to remember.

Dr. Lavelle said that the 3d was written before the 4s because the 3d subshell has less energy than the 4s subshell.

[philipraj_1F]

It technically doesn't change what is being said, but they want 3d before 4s to arrange it in increasing energy.

[PranaviKolla2B]

In what situation could we order the 4s orbital before the 3d? (just for my own knowledge because I know that we should order the 3d before the 4s in this class)

[ASetlur_1G]

4s is lower in energy if there are no electrons in the 3d level. Therefore, the 4s level is filled up before 3d because 4s is lower in energy. However, once electrons enter the 3d state, 4s becomes higher in energy than 3d and is then written after 4s.