Why is 4s before 3d for electron configurations?

1s 2s 2p 3s 3p 4s 3d 4p
Why do electron configurations have 3d orbitals out of order from the rest?
Here we break down how radial and angular nodes create a situation where 3d orbitals experience a greater repulsion from other electrons than 4s electrons do. This is in spite of the fact that 4s have a further average distance from the nucleus.
The end result is that 4s and 3d orbital states have similar energies and this creates scenarios where the lowest energy configuration depends on a myriad of factors (nuclear charge, quantum spin pairing, etc.)
This also can help explain why some of the exceptions to Aufbau are seen such as Cr being a 4s1 3d5 instead of 4s2 3d 4.

If you are interested in this video, you might like my new book Teaching Introductory Chemistry https://www.amazon.com/dp/B0B6XQBC6T