Unexpected electron filling orders

[MeenaAttota]

I also wanted a little more clarification on why certain orbitals (e.g., 4s and 3d) have unexpected electron filling orders, and how this impact chemical properties in general?

[Sahra Sutaria]

Hi!
A few factors play into unexpected electron filling orders include: shielding, electron-electron repulsion, and energy levels relative to the orbitals. The 4s orbital actually has a lower energy than 3d orbitals, meaning it has to be filled before 3d, despite higher principal quantum number. The shielding effect causes the 3d orbitals to feel more repulsion raising its energy.

Hope this helps!