Pauli Exclusion Principle :
Pauli formulated the exclusion principle which states that "No two electrons in an atom can have the same set of values of all four quantum numbers." It means that, each electron must have unique values for the four quantum numbers (n, l, m and s).
For the lone electron present in hydrogen atom, the four quantum numbers are: n = 1; l = 0; m = 0 and s = +½. For the two electrons present in helium, one electron has the quantum numbers same as the electron of hydrogen atom, n = 1, l = 0, m = 0 and s = +½. For other electron, the fourth quantum number is different i.e., n = 1, l = 0, m = 0 and s = –½.
As we know that the spin quantum number can have only two values +½ and –½, only two electrons can be accommodated in a given orbital in accordance with Pauli exclusion principle. Let us understand this by writing all the four quantum numbers for the eight electron in L shell.
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