The order in which subshells are filled

[EvaLi_3J]

Can anyone please explain why some subshells in higher energy levels are filled earlier than those in lower energy levels?

[vpena_1I]

Are you talking about cases like chromium? I know half-full (d5) and full (d10) d sub shells have lower energy than if the electrons were spread out in different, incomplete subshells. It's the symmetry that results in a more stable electron configuration.

[Mariepahos4D]

Energy levels are always filled from Lower to higher. However, the 3d state doesn’t become lower in energy until after the 4s state is filled and there are electrons in the 3d state as well just due to the arrangement of electrons and electron repulsion

[Brandon Valafar]

If you're referring to the higher energy situation with 3d and 4s, then the rule is that if you can change from d9s2 to d10s1, or from d4s2 to d5s1, you must do so.