Electron Configurations

[salvadorramos3k]

When writing the electron configurations, does 3p come before or after 4s? In a video I watched it said that 3d came after the 4s, but that doesn't make sequential sense to me and I was wondering if anyone could explain this to me? Also, in discussion my TA was saying something about the different energy levels and how one has a lower energy than the other. Can anybody also explain what that means? Thank you :)

[Sophia Shaka 3L]

3p comes before 4s, the logical sequential order. The d subshell is where things begin to get tricky. This is because the 4s subshell fills first, because it is much smaller and therefore has lower energy making it the optimal (experimentally observed) configuration. However, once the 3d subshell begins to fill, it actually becomes the lower energy orbital and this is why you have the election configuration of Ca is [Ar] 4s2 but Sc is [Ar] 3d1 4s2.

[805097738]

3p comes before 4s because you fill up the orbitals with lower energy levels first and 3 id lower than 4