Writing Electron Configuration

[Tammy Shen 2L]

In high school I remember learning to write electron configurations by following the s, d, p order (ex. 4s2 3d10 4p5 for Bromine), but I see in the textbook hw problems they'll write 3d before 4s. I also remember Dr. Lavelle mentioning how the 3d is lower than 4s in class. What is the reason for this? Why do d block orbitals have lower energies than the s blocks?

[Ella Henrickson 3I]

The energy is more related to the subshell with E_n being equal to -hr / n^2 , so when you rewrite the electron configuration in order of energy level the 3d would come before the 4s.

4s is filled first because it is initially lower in energy, but once the 3d block starts getting filled it switches back to a numerical order in the electron configuration.