In the case of a helium atom with two electrons in the 1 s orbital, the Pauli Exclusion Principle requires that the two electrons differ in the value of one quantum number. Their values of n, l, and m l are the same. Accordingly they must differ in the value of m s, which can have the value of + 1 ⁄ 2 for one electron and − 1 ⁄ 2 for the other."