Why does the 5s sub-level fill before any electrons are added to the 4d sub-level?

Answer 1

The 5s sub-level orbitals are lower in energy that the 4d sub-level so they are filled first..it is not a large difference in energy, just enough to make it significant.

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f

fill in this order due to energy values: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p....etc

I hope this helps =)

Answer 2

Because the addition of those last electrons to 5s results in an atom of lower energy than if they were added to 4d. To put it another way, if you evaluate the wavefunctions, that's the answer you get.

Some of the apparently strange orders happen because of electron-electron interactions. I'll do some extra work to climb to the 5th floor because the 4th floor is already too crowded.