Why are the sublevels within a principal level split into different energies for multi-electron atoms but not for hydrogen atom?

Kindly check the explanation section.

Explanation:

Hydrogen in the periodic chart/table is the first element on the periodic table with binding energy of 0.000± 0.0000 keV, Isotopic mass of 1.007825 u, one or single proton and a spin value of half(1/2).

The first thing to note for the fact that hydrogen's sublevels within a principal level does not split into seperate or different energies is that THERE IS A VACANT GROUND STATE IN HYDROGEN ATOM. With this, it makes it impossible for the electrons that are inside to get infiltrated which is the case with the atoms with multielectron.

NB: Penetration is done by electrons outside.

the atomic mass is the bottom number on the element.

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