d Groups 3–12 (transition metals) — d subshell filling
f Lanthanides & actinides — f subshell filling
Column notes
Valence config
outermost subshells only — the chemically active electrons (e.g. 2s²2p⁴ for O)
Anomaly ★
Aufbau / Madelung exception — 10 d-block elements (Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au) whose ground-state configuration deviates from the n+ℓ filling rule, gaining extra stability from a half-filled or fully filled d subshell (e.g. Cr: [Ar] 3d⁵4s¹ instead of [Ar] 3d⁴4s²). Several lanthanides and actinides are also irregular, but follow a more tangled pattern and are not flagged here.