The energy of orbitals

[Cecilia Lei 3K]

Lavelle mentioned that the energy of empty 3d orbital is actually higher than the energy of 4s, but after element 20, the energy of 4s will be higher than that of 3s. Can someone explain why?

[Alexandra Lu 2J]

I believe it is because, when empty, 3d orbitals are slightly farther from the nucleus than 4s orbitals (see image below), which is why 4s gets filled first. However, after element 20, once the 3d orbitals begin getting filled, the electrons in those orbitals repel the 4s subshell electrons, pushing them farther away from the nucleus. As you know, energy increases with distance from the nucleus, which is why the 4s orbitals end up being higher in energy than the filled 3d orbitals.

[Helen Ringley 2E]

So when I was in high school we just memorized an order of terms like a formula for electron configurations, does that "formula" just change based on the location on the periodic table? Like for some elements it goes 3d4s and for some it goes 4s3d? That's what I got from Lavelle's lecture yesterday but that kind of confused me.

[Alexandra Lu 2J]

Is the formula 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d…? Then yes, you can write correct electron configurations by memorizing this, but you must also memorize the exceptions, such as in the table I've linked below. The explanation for these exceptions is that a full or half-full subshell confers the most stability on an atom. Furthermore, the 4s subshell so easily donates an electron not only because the 4s and 3d orbitals are close in energy, but also because 4s orbitals after element 20 experience electron repulsion and end up being, as I mentioned in my previous reply, higher in energy.