atom, many electron The electronic configuration of many electron atoms are governed by Pauli's exclusion principle, which states that no two electrons can have the same set of quantum numbers. Pauli's exclusion principle imposes restriction on maximum number of electrons that a shell or a subshell can possess. In many electron atom, the electrons occupy different quantum states, filling from the lowest energy state upwards. The relative energy of the orbitals, and the maximum number of electrons each orbital can accommodate is shown in fig. a14. Notice the position of 3d,4d,4f and 5d orbitals in the figure. The gaps in the energy occur for Z= 2,10,18,36,54 and 86 which are the noble gases. The gaps correspond to complete filling of p subshell ( with exception of He, Z=2). In Appendix IV the electronic configuration of all the elements are listed.

Distribution of electrons in the subshells is governed by Hund's rule, according to which in p, d, and f subshells the electrons with same spin should be maximum. This reduces the electron-electron repulsive energy. Consider for example carbon (Z=6). Its the electronic configuration is, 1s²2s²2p². The electron spin distribution in p state is ­ ­ . For nitrogen, oxygen, fluorine and neon the electron spin distribution is shown in Table aII. In first transition series also, a similar way of filling occurs (see Table aIII). The inner electrons are always paired with opposite spins. Therefore they do not contribute to the spin angular momentum of the atom. The electrons in the outermost subshell which are not oppositely oriented determine the spin, and the magnetic dipole moment of the atom.