Chromium and copper are recognized as the primary exceptions to the Aufbau Principle, which governs the order of electron filling in atomic orbitals. Current understanding suggests that these are the only notable exceptions, as extensive experimental validation has been conducted for other elements, indicating no additional deviations. The exceptions arise because chromium and copper achieve greater stability by promoting an electron from the 4s orbital to the 3d orbital, resulting in half-filled and fully filled configurations, respectively. This behavior is also observed in other elements within their groups, such as molybdenum and silver. The definition of the Aufbau Principle can influence interpretations of these exceptions, and further reading on quantum mechanical explanations and ionization potentials may provide additional insights into orbital behavior.
