electron configuration clarification

[Seungri_Han_2J]

I know that for all the elements after atomic number of 20, 3d oribital comes before 4s oribital. I am still so confused as to how it works. How come it is bot 4s2 3dn? what is the reason?

[Laine Phillips]

The 3/4 refers to the n which is the energy level, thus, even though the periodic table has them out of order when writing electron configuration the energy levels need to be in order, so 3d comes before 4s.

[Jasmine Herring 3G]

I believe this is because the 4s orbital has lower energy than the 3d orbital. In addition, I think someone mentioned that the 3d orbital fills up with electrons first because of this.