Electron Configuration Confusion [ENDORSED]

[Hannah_Chao_1D]

According to the Aufbau Principle, electrons fill up lower energy orbitals first. For example the electrons fill the 4s orbital then the 3d orbital since the 3d orbital has more energy than the 4s orbital. However, the course reader says that after the 4s orbital state/orbital occupies and electrons enter 3d state, then 3d state is lower in energy than 4s. Why does the 3p orbital have less energy after the 4s orbital is filled?

[Cliff Danza 3F]

I think the part of the course reader you are referring to is this:

After 4s state/orbital occupied and e- enter 3d state, then 3d state is lower in energy than 4s

The reason for this is complicated, but here is a graph I found that illustrates what is going on with the energy levels:

You can see at n=3 the red line represents 3d, and then at n=4 the blue line represents 4s. Right around element 20, they cross, and 4s has less energy than 3d. But once 4s is filled, 3d has lower energy (right at Sc). Again, from what I've read the reasons are complicated, but hopefully this gives a clearer picture as to what is happening at the switch.

[Joslyn_Santana_2B]

Will we have to know how to write electron configurations for Quiz 1?

I am confused on that topic, and the connections of the diagrams to the atom's structure.

[sophianguyen_1N]

No, electron configurations will not be on this quiz. They will most likely appear on the next one though!