Transition metals
The transition metals (d-blocks of periods 4 to 7) are characterised by the progressive filling of the (n−1)d subshell. They exhibit several oxidation states, form coloured complexes and are often excellent catalysts.
This is the largest family in the table (40 elements) and concentrates the metals of historical and industrial importance: iron, copper, gold, silver, platinum, titanium, zinc, nickel, chromium. Their coordination chemistry is extraordinarily rich: coloured ions in solution (blue Cu²⁺, yellow Fe³⁺, green Cr³⁺, pale pink Mn²⁺), magnetism (Fe, Co, Ni), superconductivity (Nb, V), homogeneous and heterogeneous catalysis.
The irregular filling of the 3d and 4s subshells explains several striking anomalies (Cr: [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², Cu: 3d¹⁰ 4s¹), driven by the extra stability of half-filled and fully filled subshells.
One often distinguishes 'major' transition metals (3d, 4d), easily exploited, from 'heavy' ones (5d, 6d), rarer but highly prized in jewellery, watchmaking and catalysis (Pt, Ir, Au, Os, Re).