🟣 This is Higher Level (HL) content.
📘 IB Definition
A transition element is a d-block element that forms at least one stable ion with an incomplete d-subshell (d¹ to d⁹).
Exclusions: Sc (only Sc³⁺ = d⁰) and Zn (only Zn²⁺ = d¹⁰)
Characteristic Properties
| Property | Explanation |
|---|---|
| Variable oxidation states | Small energy gap between 3d and 4s → can lose varying numbers of electrons |
| High melting points | d-electrons contribute to metallic bonding → stronger electron sea |
| Coloured compounds | Partially filled d-orbitals → d–d electron transitions absorb specific wavelengths |
| Catalytic activity | Variable oxidation states provide alternative reaction pathways with lower Eₐ |
| Complex ion formation | High charge density + empty d-orbitals → attract and bond with ligands |
| Paramagnetism | Unpaired d-electrons create magnetic dipoles |
🔑 The 4s-First Ionisation Rule
Although 4s fills before 3d (Aufbau), when transition metals ionise, the 4s electrons are removed first. Once the 3d is populated, its electrons repel the 4s outward, making 4s higher in energy.
Fe: [Ar] 3d⁶ 4s² → Fe²⁺: [Ar] 3d⁶ → Fe³⁺: [Ar] 3d⁵
📌 Anomalous Configurations – Cr and Cu
Cr: [Ar] 4s¹ 3d⁵ (not 4s² 3d⁴) – half-filled d-subshell is extra stable.
Cu: [Ar] 4s¹ 3d¹⁰ (not 4s² 3d⁹) – fully filled d-subshell is extra stable.
⚠️ Examiner Trap – Colour of Complexes
Don't write "colour is emitted when electrons fall down." In complexes, light is absorbed when electrons are promoted between split d-orbitals. We see the complementary colour to what was absorbed. Also: d⁰ and d¹⁰ ions (e.g. Sc³⁺, Zn²⁺) are colourless because no d–d transitions are possible.