3d vs 4s orbitals

[Marisa Gaitan 2D]

When writing electron configurations, do we always write the 3d before the 4s? Also, why does the way we fill the orbitals change when we get to the d-block since K and Ca go from 3p to 4s, but something like Sc would be [Ar]3d1 4s2? I'm confused because I'm seeing mixed explanations online, thanks!

[Nicole Huang 3F]

Hi! Think of the 3d and 4s shifting in energy levels as they are getting filled up. So for K and Ca, the 4s would fill up because 4s has lower energy than 3d; whereas, when the 3d subshell starts to be filled up, the 4s are higher energy than the 3d. Hope this helps!

[Vanessa Perez]

yes 4s will generally always come after 3d because 3d has lower energy than 4s.

[Kimiya Aframian IB]

When writing electron configurations, do we always write the 3d before the 4s? Also, why does the way we fill the orbitals change when we get to the d-block since K and Ca go from 3p to 4s, but something like Sc would be [Ar]3d1 4s2? I'm confused because I'm seeing mixed explanations online, thanks!

Hi! When writing out electron configurations, the 3 orbitals are usually before the 4 orbitals, meaning the 3d would be before the 4s. This is consistent with the 3 having lower energy than 4. Hope this helps!

[Xinying Wang_3C]

Hi, yes, 3d will always come before 4s when writing the electron configuration. But 4s has actually slightly lower energy than 3d, so when it comes to losing electrons/adding electrons, there will be a situation like 3d^1 4s^2, since the lower energy level(4s) fills up first. I know it is really confusing, it also took me some time to understand the rule.