Question

All of the subshells in a given shell have the same energy in the hydrogen atom. In a many- electron atom, the subshells in a given shell do not have the same energy. Why?

Answer 1

be cause Hydrogen is single electron sysytem and there is only one electron and one proton in he neucleus. But in many electron system protons are increasing in nucleus and electrons are increasing in different sub levels. So the inner electrons present in the lower subshells are strongly attract by nucleus and the energy of the higher subshell different compare to hydrogen atom. this is also called shielding effect of inner electrons. so the subshell order is 1s < 2s < 2p < 3s < 3p < 3d = 4s < 4p < 4d =5s < 4f < 5p and so on. But for hydrogen due to absence of electrons the order is 1s < 2s = 2p < 3s = 3p = 3d < 4s = 4p = 4d = 4f < 5s = 5p and so on.