The physical and chemical properties of each element are directly correspondent to the position, shape, magnetic field and spin of the electrons that surround the neutral atom. Electronic configuration of hydrogen and helium are presented in this figure.įigure 2: Electronic configuration of Nitrogen.įigure 3: Electronic configuration of Oxygen.įigure 4: Electronic configuration of Aluminum.Īll atoms except hydrogen have poly electron system. Therefore, the spin on these electrons differentiates them from one another.įigure 1: In an orbital filling diagram, the square represents an orbital and the arrows represent electrons of opposite spins. As portrayed in Figure 1, a hydrogen atom has only one electron but the first three quantum numbers for helium are the same since both the electrons belong to the same energy level and same spherical shape. Therefore, when two electrons are placed in close vicinity, their outer energy level must shift slightly to allow discretion. He made it clear that every electron in an atom occupies a discrete level of energy and spin. Wolfgang Pauli (1900-1958), an Austrian physicist was awarded a Nobel prize in 1945 for his exclusion principle. This means that if one electron is assigned up spin (+1/2), the other shall be assigned a down spin (-1/2) (Singh, 2011). Therefore, only two electrons can exist in the same orbital and these electrons must have opposite spin. Pauli’s exclusion principle states that in a poly electron system, no two electrons in an atom can have the same set of four quantum numbers. Pauli’s exclusion principle corresponds with the electron spin and signifies the ability of an atom to generate a magnetic field. Therefore, two electrons may have ‘n’, ‘l’ and ‘m l’ value the same but the fourth quantum number must be different. +1/2, portrayed by ↑, or –1/2, portrayed by ↓. For the two electron in the orbital, the first three quantum number(n,l,$m_l$) are same, only the spin quantum number ($m_s$) differ.Likewise, ‘m l’ determines the number of orbitals and their orientation in the subshell and ‘m s’ specifies the direction of the electron spin i.e.This can be summed up as : the maximum number of electrons in the shell with principal quantum number n is equal to $2n^2$.

It can be also stated as only two electron can exists in the orbital and these electrons must have opposite spinģp -subshell can have maximum 6 electronsģd- subshell can have maximum 10 electrons.For example, if two electrons reside in the same orbital, then their n, l, and $m_l$ values are the same, therefore their $m_s$ must be different, and thus the electrons must have opposite half-integer spin projections of 1/2 and -1/2. As per this rule,it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers: n, the principal quantum number, l, the azimuthal quantum number, $m_l$, the magnetic quantum number, and $m_s$, the spin quantum number.

According to this principle no two electrons can have same set of 4 quantum number.

The number of electrons to be filled in various orbitals is restricted by the exclusion principle.

This principle was given by the Austrian scientist Wolfgang Pauli (1926).

Pauli exclusion principle in chemistry