Since electrons are fermions, they are subject to Pauli’s exclusion principle, which states that two fermions can not occupy the same quantum state at the same time. This principle is fundamental in determining the configuration of electrons in the atoms: once a state is occupied by an electron, the next electron must occupy a different state. In an atom, the stationary states (independent of time) of electronic wave function (that is, the states that were particulars of the Schrödinger equation HΨ = EΨ where H is Hamiltonian) are called orbital, by analogy with vision classic of the electron as a particle that orbits around the nucleus. For a multielectronic atom, with x electrons, the correct expression of the wave function must consider the spatial coordinates of all x electrons at the same time. This, in mathematical terms, is expressed by the wave function Ψ = Ψ (n1, n2, n3, … nx). However, for the purposes of chemistry, a significant simplification is utilized using the so-called “orbital approximation”: that is, each electron is considered individually as belonging to a hydrogen atom and the nuclear charge Ze, which is used to calculate the relative term for the ” potential energy to be included in the Schrödinger equation, is corrected using Zeff’s effective nuclear charge. Thus, the simplified form of the wave function, used to describe a polyelectronic atom, becomes a function of the type Ψ = Ψ(n1)Ψ(n2)Ψ(n3)…Ψ(nx).
