Which principle asserts that no two electrons in an atom can have the same set of quantum numbers?

• A. Aufbau Principle
• B. Hund's Rule
• C. Pauli Exclusion Principle
• D. Electron Configuration

Answer: (C)

No two electrons in an atom can have the same set of quantum numbers. An electron’s state is described by four quantum numbers: n, l, m_l, and m_s. The Pauli Exclusion Principle states that these four numbers must be unique for every electron in the atom. Because of this, an orbital (defined by n, l, and m_l) can hold at most two electrons, and if two electrons share that orbital, their spins must be opposite (one with +1/2 and the other with −1/2). This requirement is what guarantees that each electron has its own distinct quantum-state set, which also explains why orbitals fill up with paired electrons after single occupancy in accordance with spin.

Other ideas help describe how electron configurations are built and arranged, but they aren’t the rule that restricts identical quantum states. The Aufbau principle is about filling from the lowest energy upward, and Hund’s rule describes placing electrons singly with parallel spins in degenerate orbitals before pairing. Electron configuration is the labeling of how electrons occupy orbitals, not a principle itself.