1. Chromium (Cr): [Ar] 3d⁵ 4s¹
* Expected configuration: You'd anticipate [Ar] 3d⁴ 4s² based on the Aufbau principle and Hund's rule, which prioritize filling lower energy levels and maximizing unpaired electrons in degenerate orbitals.
* Exceptional configuration: The actual configuration has one electron in the 4s orbital and five in the 3d orbitals. This is because half-filled d orbitals are more stable than partially filled ones. Having five unpaired electrons in the 3d shell provides extra stability due to exchange energy, which is a type of electron-electron interaction that favors parallel spins.
2. Copper (Cu): [Ar] 3d¹⁰ 4s¹
* Expected configuration: Based on the rules, we would expect [Ar] 3d⁹ 4s².
* Exceptional configuration: Copper exhibits a fully filled 3d shell, which is exceptionally stable. This stability overcomes the typical preference for a half-filled 4s orbital.
In summary:
* Chromium and Copper deviate from the expected electron configurations to achieve greater stability.
* Half-filled (Cr) and fully filled (Cu) d orbitals provide more stability due to exchange energy and electron-electron repulsions.
These "exceptional" configurations contribute to the unique chemical and physical properties of these elements.
