ABSTRACT
When dealing with the multielectron atoms, the Pauli exclusion principle should always be considered. It was formulated by W. Pauli in 1925 and states that no two identical fermions may be found simultaneously in the same quantum state. Interactions between electrons in a multielectron atom lead to the formation of a number of states with different energy. The number of these states depends on the number of electrons in the atom, and their energies depend on the arrangement of electrons in the available orbitals as well as on the mutual orientation of the angular and spin momenta of all electrons. Depending on the order and magnitude of the inter-electron interaction, two types of coupling can be distinguished: LS-coupling and JJ-coupling. Depending on the type of the angular momenta coupling, different sets of quantum numbers should be used for a description of the electronic states.
