Which principle states that no two electrons in an atom can have identical 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 share the same set of quantum numbers. Each electron is described by four quantum numbers (n, l, m_l, m_s), and the Pauli Exclusion Principle requires that these four values be unique for every electron. In practice, this means that within a given orbital you can have at most two electrons, and if there are two, they must have opposite spins so their spin quantum numbers differ (one +1/2, one -1/2). This rule is what prevents identical quantum numbers from repeating and underpins why orbitals fill with two electrons at most. The other ideas describe how electrons are arranged or filled (order of filling, maximizing unpaired electrons, or simply describing the configuration), but the specific prohibition of identical quantum numbers is the Pauli Exclusion Principle.