What is a transition element?
A transition element is a d-block element that forms at least one ion with a partly filled d subshell. Scandium and zinc are d-block elements but are not transition elements, because Sc³⁺ has no d electrons and Zn²⁺ has a full d subshell. The first-row transition elements are titanium to copper.
| Element | Electron configuration (atom) | Common ions |
|---|---|---|
| Titanium | [Ar] 3d² 4s² | Ti⁴⁺ |
| Vanadium | [Ar] 3d³ 4s² | V²⁺, V³⁺, VO²⁺, VO₂⁺ |
| Chromium | [Ar] 3d⁵ 4s¹ | Cr³⁺, Cr₂O₇²⁻ |
| Manganese | [Ar] 3d⁵ 4s² | Mn²⁺, MnO₄⁻ |
| Iron | [Ar] 3d⁶ 4s² | Fe²⁺, Fe³⁺ |
| Copper | [Ar] 3d¹⁰ 4s¹ | Cu⁺, Cu²⁺ |
Characteristic properties
| Property | Explanation |
|---|---|
| Variable oxidation states | The 3d and 4s electrons have similar energies, so different numbers can be lost |
| Coloured compounds | d–d electron transitions absorb visible light |
| Complex ion formation | They have empty orbitals to accept lone pairs from ligands |
| Catalytic activity | Variable oxidation states and surface adsorption: Fe in the Haber process, V₂O₅ in the contact process, MnO₂ in decomposing hydrogen peroxide |
| Paramagnetism | Unpaired d electrons |
| High density and melting point; good conductors | Strong metallic bonding by 3d and 4s electrons |
Complex ions and colour
A complex ion has a central metal ion bonded to ligands, which donate lone pairs by dative bonds. The coordination number is the number of dative bonds. Examples: [Cu(H₂O)₆]²⁺ (pale blue, octahedral), [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue), [CuCl₄]²⁻ (yellow-green, tetrahedral), [Fe(CN)₆]³⁻, [Co(NH₃)₆]³⁺.
In a complex the ligands split the five d orbitals into two sets. An electron can be promoted from the lower to the upper set by absorbing light whose energy equals the gap (ΔE = hν). The colour we see is the complementary colour of the light absorbed. Different ligands or oxidation states give different gaps and so different colours.
Choose the number of unpaired d electrons. The spin-only magnetic moment is μ = √(n(n + 2)) Bohr magnetons.
| Ion | d electrons | Unpaired (high-spin) |
|---|---|---|
| Ti³⁺ | 1 | 1 |
| Cr³⁺ | 3 | 3 |
| Mn²⁺ | 5 | 5 |
| Fe³⁺ | 5 | 5 |
| Fe²⁺ | 6 | 4 |
| Cu²⁺ | 9 | 1 |
| Zn²⁺ | 10 | 0 |
Key terms
- Transition element
- A d-block element that forms an ion with a partly filled d subshell.
- Ligand
- A molecule or ion that donates a lone pair to a metal ion in a complex.
- Coordination number
- The number of dative bonds to the central metal ion.
- Complementary colour
- The colour seen when a colour of light is absorbed.
Summary
- Transition elements form coloured compounds, complex ions, several oxidation states and catalysts.
- Ligands split the d orbitals; light is absorbed when an electron moves to the upper set.
- The colour seen is complementary to the colour absorbed.
- Unpaired electrons give paramagnetism: μ = √(n(n + 2)).