How is Palladium able to cram all its electrons into it’s first 4 valence shells, but the elements both above AND below it are forced to use their 5th shell?

Question

I understand the basics of valence shells but not enough to figure out why palladium is unique. I expected the elements immediately left of it (rhodium, ruthenium, technetium) to have fewer electrons in their 4th shell–and they do–but they also always have at least one electron in the 5th shell.

Also here’s some neat diagrams of this if anyone’s interested: https://valenceelectrons.com/category/electron-configuration/page/8/

In: 12

Answer 1

The 5s orbitals have lower energy than the 4d orbitals, despite being a higher quantum number. So for the other elements you listed, it is more energetically favorable to have electrons in the 5s. Palladium differ in that it has enough electrons to fill the 4d orbitals entirely. A filled 4d has lower energy than a 5s, so instead of 4d9,5s1 it becomes 4d10,5s0