When there are two electrons in the same orbital, they have:
(1) same spin
(2) opposite spin
(3) same or opposite spin
(4) no spin
Answer: (2) In the case of two electrons occupying the same orbital, the Pauli principle demands that they have opposite spin. 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 the exchange of the particles.
