When would 3d orbital be filled before the 4s orbital?

[EvaLi_3J]

When would 3d orbitals be filled before the 4s orbital?
And similarly, when would 4d filled before 5s? 4f before 6s?

I am just kind of confused as for what exactly is the order in which we fill our electrons into the orbitals. Obviously, there are quite some exceptions.

[Althea Zhao 1B]

I believe that past the 4s2 elements, you start filling the d orbital first. For the f orbital, you start filling it after 6s2 (Ba).

[Quynh Vo]

You usually always fill 4s before 3d (with exceptions of Cr and Cu) but I guess after both orbitals are filled, it wouldn't matter when writing it out?

[Harry Zhang 1B]

The 3d orbital will never be filled before the 4s orbital because any (n+1)s orbital is lower in energy than the nd orbital if there is no electron in the 3d orbital. However, after the 4s is filled and there is one electron in the 3d orbital, you would move the 3d orbital to the left of the 4s orbital because it is now lower in energy. Another thing that should be noted is that when the electron configuration 3d4 4s2 or 3d9 4s2 occurs, you will pull one electron in the 4s into the 3d orbital since a 3d5 and 3d10 orbital is more stable and therefore lower in energy than 3d4 or 3d9. The same thing applies to when you go beyond Xe. You would fill the 6s first and then fill one electron into the 5d and move it to the left of 6s. However, the next electron will go into the 4f orbital and the 4f orbital will be moved to the left of the 5d orbital, but these electron configurations (from La to Lr)are for the lanthanides, which is not required for the test.

[Ayushi2011]

4s is lower in energy than 3d, causing it to get filled up first.

[Siddiq 1E]

After the 4s orbital is occupied AND electrons enter the 3d state, THEN the 3d state is lower in energy than 4s, so it would be written first.

[pJimenez3F]

You usually always fill 4s before 3d except for Cr and Cu

[Victor Qiu 1C]

It would go as 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

[Jaclyn Schwartz 1I]

You always fill 4s orbital first, except with Cr and Cu. 4s has a lower energy so it fills before 3d. Just follow the order above^^ and it will work for most of the time.