4s to 3d

[peytonruiz 1H]

Can someone explain why 4s comes before 3d despite 3d being slightly lower in energy compared to 4s?

[Ivy Lu 1C]

In the cases where 4s comes before 3d, it just has to do with the charge of the atom. When n increase, the subshells are closer together and the charge of the atom can cause 4s to be filled first like with potassium and calcium.

[Yifei Wang 3G]

Overall the energy of electrons in the 3rd shell is lower than that of electrons in the 4th shell, but when it comes to the specific subshells, it's a different story. When comparing 4s and 3d, 4s indeed has lower energy.

[Michelle Lee 2E]

Overall, I'm still a little confused.
I would like to add as well the model that Dr. Lavelle always uses for describing the shells in hydrogen. (The one with the positive charge--nucleus--at the bottom of the model and horizontal lines above it to signify the rising shells. He always mentions that as the quantum number increases, the shells get closer and closer together. We also have to remember that the electrons don't revolve around the nucleus on these set shells as well.

The 4s is slightly lower in energy than the 3d [seen on page 46 of chapter 2.6].
"for multi electron atoms after atomic number 20, 4s state is higher in energy than 3d state [in friday's lecture].
Can someone explain why these two forms of information contradict? Maybe I missed something in the book.

[Leanne Wong 1H]

The reason why the two statements contradict is due to the fact that 4s orbital is lower in energy when 3d is only partially filled however when 3d is completely filled then it becomes lower than the 4s orbital.

[Rakhi Ratanjee 1D]

Leanne, I'm a bit confused on your statement. You say that when 3d is completely filled, then it is lower in energy than 4s? How can this be the case when you're adding electrons? Don't more electrons mean greater energy?