Pauli’s exclusion principle: No two electrons in an atom can have the same value for the same set of four quantum numbers. Thus if two electrons have identical values of n ℓ and m it follows from this principle that they must differ in their spins i.e., +½ or -½. This means that two electrons can occupy the same sub-level and only if their spin are opposed. Natural implication of this principle is that no more than two electrons can occupy the same atomic orbital.
