IB Definition
A transition element is a d-block element that forms at least one stable ion with an incomplete (partially filled) d-sublevel.
Scandium and Zinc Are Not Transition Metals
Scandium (Sc) and Zinc (Zn) are d-block elements but NOT transition metals under the IUPAC definition:
- \(\text{Sc}^{3+}\) configuration is \([\text{Ar}]\,3\text{d}^0\) (completely empty d-subshell)
- \(\text{Zn}^{2+}\) configuration is \([\text{Ar}]\,3\text{d}^{10}\) (completely full d-subshell)
Neither forms a stable ion with an incomplete d-subshell (\(\text{d}^1\) to \(\text{d}^9\)).
1. Variable Oxidation States
Transition elements can form ions with multiple different charges because the 4s and 3d orbitals are very close in energy. Electrons are always lost from the 4s sub-level before the 3d.
| Element | Symbol | Common Oxidation States | Config [Ar] |
|---|---|---|---|
| Titanium | Ti | +2, +3, +4 | 3d² 4s² |
| Vanadium | V | +2, +3, +4, +5 | 3d³ 4s² |
| Chromium | Cr | +2, +3, +6 | 3d⁵ 4s¹ |
| Manganese | Mn | +2, +4, +6, +7 | 3d⁵ 4s² |
| Iron | Fe | +2, +3 | 3d⁶ 4s² |
| Cobalt | Co | +2, +3 | 3d⁷ 4s² |
| Nickel | Ni | +2 | 3d⁸ 4s² |
| Copper | Cu | +1, +2 | 3d¹⁰ 4s¹ |
2. Catalytic Activity
Transition metals are highly effective catalysts because:
- Variable oxidation states allow them to gain/lose electrons easily during redox reactions (alternative pathway with lower Ea)
- Empty d-orbitals allow temporary adsorption of reactant molecules onto solid surfaces, weakening bonds
| Catalyst | Process | Type |
|---|---|---|
| Fe | Haber process (N₂ + H₂ → NH₃) | Heterogeneous |
| V₂O₅ | Contact process (SO₂ → SO₃) | Heterogeneous |
| MnO₂ | Decomposition of H₂O₂ | Heterogeneous |
| Ni | Hydrogenation of alkenes | Heterogeneous |
3. Coloured Compounds
Transition metal ions with partially filled d-orbitals absorb specific wavelengths of visible light, causing d-d electron transitions. The colour observed is the complementary colour of the light absorbed.
| Ion | Colour | Example Compound |
|---|---|---|
| Cu²⁺ | Blue | CuSO₄(aq) |
| Fe²⁺ | Pale green | FeSO₄(aq) |
| Fe³⁺ | Yellow/brown | FeCl₃(aq) |
| Cr³⁺ | Green | Cr₂(SO₄)₃(aq) |
| MnO₄⁻ | Purple | KMnO₄(aq) |
| Cr₂O₇²⁻ | Orange | K₂Cr₂O₇(aq) |
4. Formation of Complex Ions
Transition metal ions have high charge density and energetically accessible d-orbitals, allowing them to attract electron-rich ligands to form complex ions. See S3.1.9 for full details.
5. Magnetic Properties
The presence of unpaired d-electrons gives transition metal compounds paramagnetic properties; they are attracted to magnetic fields. Elements/ions with all paired electrons are diamagnetic (weakly repelled).
4s Electrons Lost Before 3d Electrons
Remember: 4s electrons are always removed before 3d electrons when transition metal atoms form positive ions. For example, \(\text{Fe} = [\text{Ar}]\,3\text{d}^6\,4\text{s}^2\), and \(\text{Fe}^{2+} = [\text{Ar}]\,3\text{d}^6\) (the two \(4\text{s}\) electrons are lost first).
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