Electronic configuration and trends
d-Block Elements (Transition Metals) · Chemistry
Study notes
Why are Cr (3d⁵4s¹) and Cu (3d¹⁰4s¹) exceptions to the Aufbau order? Step 1: Expected: Cr 3d⁴4s², Cu 3d⁹4s². Step 2: Half-filled (d⁵) and fully-filled (d¹⁰) subshells have extra stability (exchange energy). Step 3: Promoting one 4s electron to 3d gains this stability. Step 4: Same logic: Mo 4d⁵5s¹, Ag 4d¹⁰5s¹. Symmetry pays!