https://chemistry.stackexchange.com/questions/152/why-is-the-2s-orbital-lower-in-energy-than-the-2p-orbital-when-the-electrons-in

The <principal quantum number> thing fully determining energy is only true for the <hydrogen emission spectrum> for which we can solve the <Schrödinger equation> explicitly.

For other atoms with more than one electron, the orbital names are just a very good approximation/perturbation, as we don't have an explicit solution. And the internal electrons do change energy levels.

Note however that due to the more complex effect of the <Lamb shift> from <QED>, there is actually a very small 2p/2s shift even in hydrogen.


Discussion on "Why does 2s have less energy than 1s if they have the same principal quantum number?" by Ciro Santilli (@cirosantilli, 34)