3d and 4s States

[Letty Liu 2C]

I'm confused about the energy levels with 4s and 3d. How is 4s before 3d if it is a higher energy level? Could someone explain the reasoning behind this concept and how it applies to the elements we went over in class?

[SMIYAZAKI_1B]

Okay so this might sound weird, but when the electrons are not filled into these two orbitals, 3d shell actually does have a higher energy. Therefore, the electrons will fill up 4s orbitals, which are much lower in its energy. However, once the electrons have filled for both shells, the. 4s sub-shell will have higher energy (like two's power level swaps before electrons are filled and after electrons are filled). As a result, it causes the 4s shell to be filled first before 3d shell.

[SMIYAZAKI_1B]

In addition, when the valence electrons of these elements are used for chemical reactions...
Fe 1s2 2s2 2p6 3s2 3p6 3d6 4s2
Fe3+ 1s2 2s2 2p6 3s2 3p6 3d5
The electrons from 4s orbitals are used first.