Why does the 4s orbital come before the 3d?

[Mario Reyes 1C]

If I heard correctly in class, the 4s orbital comes before 3d only for elements with 20 electrons or less. Can somebody explain why?

[Joanna Pham - 2D]

I was confused about this too. I don't understand the exact reason why 3d comes before 4s when writing the electron configuration.

[Jennifer Tuell 1B]

3d is at a lower energy level than 4s therefore it comes before it in the electron configuration because it going from lowest to highest energy level.

[Nandhini_2K]

The 4s orbitals are lower than 3d.

[Joanna Pham - 2D]

I am curious,why exactly does 3d have a lower energy level than 4s?

[tmehrazar]

the 4s orbitals have lower energy than the 3d orbitals not the other way around, some things online say that s-orbitals are more penetrating than d-orbitals so i think that may be why

[MadelynNguyen1F]

I think 4s might fill up first because of electron repulsion. I was looking online and it said that was related to the fact that 4s fills up before 3d.

[Alejandro Salazar 1D]

The electron configuration for Cr is different, by having the 3d come first before the 4s because having two electrons in the 4s makes the orbital more unstable because of electron repulsion