No two electrons in an atom can have identical quantum numbers

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

Answer: (A)

The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers (n, l, m_l, m_s). This means each electron must occupy a distinct quantum state. In practice, electrons can share the same orbital's first three quantum numbers (n, l, m_l), but their spins must differ, giving opposite spin quantum numbers (m_s = +1/2 and m_s = -1/2). This is why an orbital can hold at most two electrons. Other rules like Aufbau and Hund’s Rule describe how those states are filled across an atom, but the fundamental limit that prevents identical quantum numbers comes from this principle.