Which principle states that no two electrons can have the same set of quantum numbers?

• A. Aufbau principle
• B. Hund's rule
• C. Pauli's Exclusion Principle
• D. Heisenberg Principle

Answer: (C)

Pauli Exclusion Principle states that no two electrons in an atom can have the same four quantum numbers (n, l, m_l, m_s). Because each electron is described by those numbers, an orbital can hold at most two electrons, and those two must have opposite spins (one with m_s = +1/2 and the other with m_s = -1/2). This rule is what makes electron configurations unique and explains why electrons fill different orbitals and pair up only after each available orbital in a subshell has one electron. Other rules like Aufbau describe the order in which orbitals are filled by energy considerations, Hund's rule describes how electrons populate degenerate orbitals with maximum unpaired spins, and the Heisenberg principle concerns fundamental limits on measuring position and momentum; none of these by themselves impose the unique-quantum-number constraint, which is exactly what this statement captures.