Explanation:

The Pauli exclusion principle is the quantum mechanical principle that no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles. For example, no two electrons in a single atom can have the same four quantum numbers; if n, l, and ml are the same, must be different such that the electrons have opposite spins.