The s orbital has a higher probability to be closer to the core and feels a larger attraction than the p orbital. On average, the s orbital is further away and in addition, the p orbital has a repulsive potential of l(l+1)h²/2mr². What is the usual energy difference between these levels and how is it measured, considering that the transitions are highly forbidden? Does quantum electrodynamics separate s from p? In Lamb shift, I think it does not, but how about in multielectron atoms? In real life, if hydrogen was not a molecule but an atom, is s lower than p? And is it the opposite in all multielectron atoms? Is there no Hund's rule for this?