In polyelectronic atoms, the orbitals of a given principal quantum level are not degenerate. Inpolyelectronic atoms, the energy order of the n = 1, 2, and 3 orbitals are (not to scale):In general, the lower the n value for an orbital, the closer on average the electron can be to thenucleus, and the lower the energy. Within a specific n value orbital (like 2s vs. 2p or 3s vs.3p vs. 3d), it is generally true that Ens < Enp < End < Enf.To rationalize this order, we utilize the radial probability distributions. In the 2s and 2pdistribution, notice that the 2s orbital has a small hump of electron density very near thenucleus. This indicates that an electron in the 2s orbital can be very close to the nucleus someof the time. The 2s electron penetrates to the nucleus more than a 2p electron, and with thispenetration comes a lower overall energy for the 2s orbital as compared to the 2p orbital.In the n = 3 radial probability distribution, the 3s electron has two humps of electron densityvery close to the nucleus, and the 3p orbital has one hump very close to the nucleus. The 3sorbital electron is most penetrating, with the 3p orbital electron the next most penetrating,followed by the least penetrating 3d orbital electron. The more penetrating the electron, thelower the overall energy. Hence the 3s orbital is lower energy than the 3p orbitals which islower energy than the 3d orbitals.