Chromium (Cr) and copper (Cu) are the primary exceptions to the Aufbau Principle, which governs the order of electron filling in atomic orbitals. Current experimental evidence suggests that no other elements deviate from this principle, as extensive testing has confirmed the validity of the Aufbau Principle across the periodic table. The exceptions occur because Cr and Cu draw one electron from the 4s orbital to achieve a half-filled or fully filled 3d orbital, resulting in a more stable electron configuration. This behavior is also observed in other transition metals such as molybdenum (Mo) and silver (Ag).
PREREQUISITESChemistry students, educators, and researchers interested in quantum chemistry, atomic structure, and the behavior of transition metals in relation to the Aufbau Principle.