Electron Configurations

[caitlin_oberman_2G]

Does anyone know that when we write electron configurations if we we are always supposed to write the equations in increasing numerical order? For example Iron (Fe):

[Ar] 3d^6 4s^2 compared to [Ar] 4s^2 3d^6

[Yesenia Munoz 1G]

I believe we are supposed to write it in increasing numberical order.

[TiengTum2D]

I believe we have to write it in increasing numerical order because the d orbitals become lower in energy once the s orbitals have been filled. Let's take iron, Fe, for example. Once the 4s orbitals have been filled and the electrons start to occupy the 3d orbitals, the 3d orbitals become lower in energy, which is primarily the reason why we have to write it before the 4s orbitals. When forming ions, the 4s electrons will be the first ones to be lost since it is the furthest away from the nucleus and in the highest energy level.