Order of Filling 3D and 4S Orbitals

[Samantha Pedersen 2K]

I was reviewing this question: "Explain the order of electron filling in atoms of K, Ca, and Sc."

My understanding is that the 4S subshell is lower in energy than 3D when it is empty, so it fills up first in K and Ca. But would the 4S or 3D subshell fill first in scandium, and why?

Thank you!

[Luveia Pangilinan 1A]

The transition metals of 3d orbital actually become lower than the energy of 4s! It's also to avoid electron repulsion :D

[Faaizah Arshad 1H]

I am still confused about this. For K and Ca, is the 4s subshell always lower energy, so 4s fills first? For Sc and all transition metals, is the 3d subshell always lower in energy, so 3d fills first?