CHEMISTRY COMMUNITY

Hi!

Can someone explain why 4s is sometimes written before 3d and why after Z=20, it's written after 3d?

I have in my notes that after 4s state/orbital is occupied and electrons enter the 3d state, then the 3d state is lower in energy than 4s, but for multi electron atoms after atomic number(z) 20 4s is in a higher state of energy than 3d. The atomic number(z) 20 is for Ca, which is when the 4s orbital is occupied. Period 5 has the same trend, where the 4d state is lower in energy than the 5s stats after 2 electrons have occupied the 5s state.
I hope this helps!

4s is a lower energy orbital than 3d and you want to have the lowest energy configuration possible, so you want to fill up the 4s shell before the 3d. There are two (i think) exceptions which the textbook covers, so i'd check that out!

Awesome thanks! And these are all based on experimental findings right?

In the lecture of 10/26 Dr. Lavelle says that you can figure it out based on the periodic table and by calculating the numbers using Pauli's Exclusion Principle and Hund's rule. I think it should be fine if you just remember that the 4s goes before the 3d.