No two electrons in the same atom can have the same set of four quantum numbers is the

• A. Pauli exclusion principle
• B. Hund's rule
• C. Aufbau principle
• D. Noble gases

Answer: (A)

The statement expresses the Pauli exclusion principle. In atoms, electrons are described by four quantum numbers: n, l, m_l, and m_s. The first three specify the energy level, sublevel, and orbital, while m_s denotes the spin. Because electrons are fermions, no two electrons can share the exact same set of all four quantum numbers. This means an orbital can hold at most two electrons, and those two must have opposite spins (one with spin up and one with spin down). That’s why you can have two electrons in a single orbital, but not a third. The other ideas listed describe different rules or concepts: Hund’s rule describes how electrons fill degenerate orbitals to maximize unpaired spins; the Aufbau principle describes the order in which orbitals are filled by increasing energy; noble gases refer to elements with filled shells, not a principle about quantum numbers.