Thus, the Pauli's exclusion principle may also be stated as "An orbital can have maximum of two electrons and these two must have opposite spin". The maximum number of electrons in each main energy level can be predicted by extending this principle. The tabular column given below the various combinations of quantum numbers for electrons in n = 1 to n =3 levels.
Pauli's exclustion principle
This principle was proposed by Pauli in 1952. It states that no two electrons in an atom can have same values for all the four quantum numbers. Thus, in the same atom, two electrons may have the same values for three quantum numbers but the fourth must be different. Electrons having the same value of n, l and m are said to belong to the same orbital. For instance, consider K shell, i.e. n =1. The electron will have only one value of (l) which is l = 0 and one value of m, which is m = 0 but it can have two values of s, either + ½ or - ½. This means that although n, l and m are the same for the two electrons but their spin quantum numbers are different. Thus, an orbital can have maximum of two electrons. Moreover, if an orbital has two electrons, they must be of opposite spin. For one s = + ½ and for other s = - ½ . For e.g., the set of four numbers for the two electrons present in 1s-orbital are: