Electron Configuration

[Rita Chen 1B]

Can someone please clarify what Dr. Lavelle said about putting 3d in front of 4s? Thank you!

[Sebastian Lee 1L]

Basically, when writing electron configurations you must write in order of lowest energy to highest energy. In K and Ca, the 4s subshell is of a slightly lower energy than the 3d subshell so electrons occupy that space first. However, when Z>20 (you enter the d-block), the 3d subshell actually shifts down to a lower energy level than the 4s subshell. So we need to write 3d before 4s even though 4s if already full for the most part.