Zeff2 En = - 13.6 -------- eV n2Note we use Zeff instead of Z to mean that we have to modify the atomic number to get an effective atomic charge for the nucleus. Since we are dealing with approximate values, one may use Z directly. The 1s orbital energy level is -13.6 eV for hydrogen atoms, measured as the ionization energy of H.
Thus, for the quantum number n = 1, the energy level for 1s of He is approximately - 54 eV.
Similarly, the 1s energy level for F is - 1101 eV.
The 2s and 2p energy levels for He is approximately - 13.6 eV, which is simlar to that of 1s orbital of H.
Thus, the 2s energy level for Li is approximately -6 eV. However, for multi-electron atoms, the p-subshell and s-subshell have different energies due to penetration. At this level, we cannot be precise about it, but simply think that the 2p orbitals are at higher enrgy than the 2s orbital.