The 4s and 3d subshells: Which one fills first in progressing through the periodic table and which one fills first in any particular atom? #chempaperaday 266

Electron configuration of transition metals sometimes get complicated due to some "irregularities". If you have NEVER noticed this, look them up in an inorganic textbook. This paper doesn't really give you anything new if you already know the underlying reason for those configurations. However, the authors did a computational experiment to show us which orbitals are more stable upon adding an electron. So, that's useful in terms of teaching.

The summary is that  "there is no scientific reason to write the electron configuration of transition elements as [Ar] 4s 3d and the correct form is [Ar] 3d 4s."

http://link.springer.com/article/10.1007/s10698-016-9249-0