As explained above, electrons of the He atom having the same value of three quantum numbers (n, l, m), have different values of spin quantum number (s). The ‘+’, and ‘-‘ sign in spin quantum number signifies the clockwise and anticlockwise spin of the electron. Thus, by the principle of quantum numbers and the Pauli exclusion principle, the quantum numbers of these two electrons of the He atom are: As shown in the figure below, the helium (He) atom has two electrons in its 1s orbital.

Pauli Exclusion principle ExampleĪs an illustration of Pauli’s Exclusion Principle, consider neutral helium (He) atom. This indicates that if one electron is allocated to spin up (+1/2), the second electron must be assigned to spin down (-1/2).

Because of the fact that an orbital can only hold two electrons at a time, the two electrons must have opposing spins. Pauli’s Exclusion principle states that no two electrons in an atom or molecule can have the same quantum numbers. According to this principle, “ no two electrons in an atom can have the same set of all four quantum numbers.” Pauli Exclusion Principle definition Pauli was awarded Nobel Prize in Physics in 1925 for his significant contribution through the discovery of a new natural rule, the exclusion principle, or Pauli’s principle. There are only three possible combinations of ( n, l, m l): (2, 1, 1), (2, 1, 0), and (2, 1, −1).ī Because m s is independent of the other quantum numbers and can have values of only +½ and −½, there are six possible combinations of ( n, l, m l, m s): (2, 1, 1, +½), (2, 1, 1, −½), (2, 1, 0, +½), (2, 1, 0, −½), (2, 1, −1, +½), and (2, 1, −1, −½).Ĭ Hence the 2 p subshell, which consists of three 2 p orbitals (2 p x, 2 p y, and 2 p z), can contain a total of six electrons, two in each orbital.Pauli Exclusion principle was proposed by an Austrian scientist, Wolfgang Pauli who studied atomic spectra of elements in 1945. Add together the number of combinations to predict the maximum number of electrons the 2 p subshell can accommodate.Ī For a 2 p orbital, we know that n = 2, l = n − 1 = 1, and m l = − l, (− l +1),…, ( l − 1), l.Build on these combinations to list all the allowed combinations of ( n, l, m l, m s).List all allowed combinations of ( n, l, m l). List the quantum numbers ( n, l, m l) that correspond to an n = 2 p orbital.List all the allowed combinations of the four quantum numbers ( n, l, m l, m s) for electrons in a 2 p orbital and predict the maximum number of electrons the 2 p subshell can accommodate.Īsked for: allowed quantum numbers and maximum number of electrons in orbital An electron behaves like a magnet that has one of two possible orientations, aligned either with the magnetic field or against it. These are described by a fourth quantum number ( m s), which for any electron can have only two possible values, designated +½ (up) and −½ (down) to indicate that the two orientations are opposites the subscript s is for spin.

Pauli exclusion principle in chemistry