Principle Quantum Number in D-Block [ENDORSED]

[isauramora3K]

Why does the principle quantum number drop down by one when you reach the D-block?

[Nancy Dinh 2J]

The d-block actually belongs to the shell above, but because the energy difference between each principal level gets smaller and smaller as n increases, the electrons start to fill in the ns orbitals before they start filling in the (n-1)d orbitals.

(I like to think of it as the electrons getting confused because of the very small energy difference. Sort of like trying to see the difference between the colors peach and salmon.)

[Vincent Kim 2I]

You start filling the 3d orbital after 4s because the 4s orbital generally has lower energy than the 3d orbital.

[Justin Lai 1C]

Why do electrons fill up the 4s shells before the 3d shells in Potassium and Scandium if 3d is lower energy?

[Nathan Tu 2C]

The simple answer is that the energy difference is that the energy difference between 3d and 4s is so small, filling up 4s first happens to be more stable. With exception of Cu and Cr and elements similar to those.