4s before 3d

[Rebecca Epner 4A]

Can someone explain exactly why in some instances 4s come before 3d? And in what cases does this occur?

[GFolk_1D]

Hi here's a diagram of Aufbau's Principle, hope this helps clarify!

[Abigail Sanders 1E]

4s will come before 3d as it has a slightly lower electrostatic energy. This allows the subshell to be filled before 3d. Once 4s is full, then 3d has the lower energy so it fills with electrons.

[Khushboo_3D]

The reason for this is the difference in energies. According to Aufbau's principle, the 4s shell has a lower energy than the 3d shell and hence it gets filled first.

[805394719]

The 4s orbital has lower energy than the 3d orbital when it is unoccupied by electrons. Because electrons always fill the lower energy level first, 4s orbital will be filled before 3d. After 4s orbital has been occupied by electrons, the 3d orbital will be filled which will shield the 4s electrons from the electrostatic interaction of the nucleus. When 3d orbital is filled, the 4s orbital has a higher energy so when an electron is pulled from the valence shell, the 4s orbital will lose an electron first, before 3d orbital does. Thus, when ionizing an element by removing an electron, the 4s orbital will lose electrons first.