Electron Configuration

[altaing]

Why do we write the s orbital after the d orbital in electron configuration when dealing with elements in the d-block?

[PranithaPrasad]

I think it's because the oribital are in order of increasing energy, so we would write 3d before 4s since 3d is a less energy orbital than a 4s orbital.

[aTirumalai-1I]

To clarify, the textbook says that when the 3d subshell is empty, its energy is greater than than that of the 4s subshell; thus for elements like Potassium (K), the s orbital is filled first. However, when the 3d orbital is occupied by one or more electrons, its energy becomes lower than the energy of the 4s orbital, which is why when writing out electron configurations, an occupied 3d orbital is written before an occupied 4s orbital. Hope that made sense!