Topic overview

Start with the big picture

Half-filled configurations such as p³, d⁵, and f⁷ benefit from exchange energy associated with parallel-spin electrons in degenerate orbitals. The number of possible exchanges rises with the number of parallel spins, with the greatest exchange stabilization at half filling. Fully filled subshells, including p⁶, d¹⁰, and f¹⁴, also benefit from spherical symmetry, which reduces electron–electron repulsion. These energy effects can favor configurations such as chromium’s [Ar] 4s¹ 3d⁵ and copper’s [Ar] 4s¹ 3d¹⁰ over simpler expected arrangements. The effect is context-dependent: subshell energy gaps can outweigh the stabilization, and promoted electrons may be removed first during ionization. The lesson connects these configurations with Hund’s rule and broader periodic and magnetic behavior.