Which principle states that no two electrons can have identical sets of all four quantum numbers?

• A. Aufbau Principle
• B. Pauli Exclusion Principle
• C. Hund's Rule
• D. Dalton's Law

Answer: (B)

This question tests the Pauli Exclusion Principle. It states that no two electrons in an atom can share the same four quantum numbers (n, l, ml, ms). Since the spin quantum number ms can have only two values, an orbital defined by n, l, and ml can hold at most two electrons, and those two must have opposite spins. This is what prevents identical electronic states and forces electrons to occupy distinct quantum states as atoms are built up. For context, the Aufbau principle describes the order in which orbitals are filled, Hund’s rule explains maximizing unpaired electrons in degenerate orbitals, and Dalton’s law relates to gas mixtures rather than atomic structure.