Exceptions to the Aufbau Principle for Transition Metals

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The Aufbau principle states that electrons fill orbitals starting at the lowest available energy states before filling higher states. However, this principle is a guideline, and several exceptions occur, particularly in the transition metals (d-block elements).These exceptions are driven by the enhanced stability of half-filled (d5) and fully-filled (d10) d-subshells.The most common and important exceptions to know are in Period 4:Chromium (Cr, Z=24)Expected Configuration (by Aufbau): [Ar] 4s² 3d⁴Actual (Observed) Configuration: [Ar] 4s¹ 3d⁵Explanation:An electron is "promoted" from the 4s orbital to the 3d orbital. The energy cost of this promotion is more than compensated for by the significant stability gained from achieving a half-filled d-subshell (3d⁵). In this configuration, every d-orbital has one electron, maximizing spin (Hund's Rule) and creating a stable, symmetrical arrangement.Copper (Cu, Z=29)Expected Configuration (by Aufbau): [Ar] 4s² 3d⁹Actual (Observed) Configuration: [Ar] 4s¹ 3d¹⁰Explanation:Similar to chromium, an electron is promoted from the 4s orbital to the 3d orbital. This promotion results in a fully-filled d-subshell (3d¹⁰) and a half-filled s-subshell (4s¹). The exceptional stability of the completely full d-subshell makes this s¹ d¹⁰ configuration much more favorable than the s² d⁹ configuration.Other ExceptionsThis trend of prioritizing d⁵ and d¹⁰ configurations continues and becomes even more common in the heavier transition metals (Periods 5 and 6):Molybdenum (Mo, Z=42): [Kr] 5s¹ 4d⁵ (follows the Chromium pattern)Silver (Ag, Z=47): [Kr] 5s¹ 4d¹⁰ (follows the Copper pattern)Palladium (Pd, Z=46): A unique case where both s-electrons are promoted: [Kr] 5s⁰ 4d¹⁰Gold (Au, Z=79): [Xe] 6s¹ 4f¹⁴ 5d¹⁰ (follows the Copper pattern)

Exceptions to the Aufbau Principle for Transition Metals

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