Chapter 18 — Chapter 18 — Transition Metals

First row transition elements are d-block elements having partially filled d-orbitals in any of their stable oxidation states. They are called transition elements because they show transitions between variable oxidation states. Though the d-block covers groups 3 to 12, zinc and scandium are not classified as transition elements: Zn forms only Zn²⁺, which has a complete 3d subshell ([Ar] 3d¹⁰ 4s⁰) (and Sc forms only Sc³⁺ with an empty 3d). Configurations follow the Aufbau principle except chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹), which gain the stability of a half-filled and completely filled d-subshell; this makes their electron cloud more symmetrical. In forming ions, 4s electrons are lost first: e.g. Fe [Ar] 3d⁶ 4s², Fe²⁺ [Ar] 3d⁶, Fe³⁺ [Ar] 3d⁵; Cu²⁺ [Ar] 3d⁹.