Midterm Question

[elizabethrojas1G]

So on the midterm one of the questions was "Explain the order of electron filling in atoms of K, Ca, and Sc. Give the abbreviated electron configurations to illustrate your answer."
In the explanation, it said that for K and Ca, the 4s orbital was lower in energy than 3d but in Sc 3d was lower in energy than 4s, how do we know when either situation applies? Would this also apply for every row with different scenarios, so like would 4d be less than 5s in one case and greater than 5s in another?

[Chem_Mod]

When the 3d orbital does not have any electrons in it, it is higher in energy than the 4s (as in the case with K and Ca). However, as the 4d orbital is filled, the presence of the electrons stabilize it enough to be lower in energy than the 4s as in the case with Sc.