3d Electron Delocalisation in Transition Elements
This is Higher Level (HL) content.
Transition metals (d-block, excluding Sc and Zn) have significantly higher melting points than s-block metals. This is due to the participation of d-electrons in the delocalised electron sea.
Transition Element
A transition element is a d-block element that forms at least one stable ion with a partially filled d-sublevel.
Why Zn and Sc are excluded: Zn only forms Zn²+ (3d¹&sup0; - full d-sublevel). Sc only forms Sc³+ (3d&sup0; - empty d-sublevel). Neither has a partially filled d-sublevel in any stable ion.
Why Transition Metals Bond More Strongly
Why Transition Metals Have Extra-Strong Metallic Bonds
- Transition metals can delocalise both their s- and d-electrons into the electron sea
- More delocalised electrons → greater electron density in the sea
- Higher cation charge + more electrons → stronger electrostatic attraction
- Transition metal cations are also smaller (greater effective nuclear charge) → electrons are held more tightly
Comparison: s-Block vs d-Block
| Na (s-block) | Fe (d-block) | |
|---|---|---|
| Delocalised electrons | 1 per atom (3s¹) | Multiple (4s + 3d electrons) |
| Cation charge | +1 | +2 or +3 |
| Ionic radius | Larger (102 pm) | Smaller (77 pm for Fe²⁺) |
| Melting point | 98 °C | 1538 °C |
Zinc and Scandium Exception Details
Do not say d-block metals have "more metallic bonds." The correct explanation is: more electrons are delocalised and the cation charge is higher, resulting in a stronger electrostatic attraction between the cations and the electron sea.
Comparing Melting Points: Na vs Mg vs Fe
Na (s-block): 1 delocalised electron, +1 cation, large ionic radius. Weakest metallic bond.
Mg (s-block): 2 delocalised electrons, +2 cation, smaller ionic radius. Moderate metallic bond.
Fe (d-block): Multiple s- and d-electrons delocalised, +2 or +3 cation, smallest ionic radius. Strongest metallic bond due to d-electron participation.
Predicted order: Na (98 °C) < Mg (650 °C) < Fe (1538 °C)
Other Properties of Transition Metals
Transition metals share several characteristic properties beyond high melting points:
- Variable oxidation states: d-electrons can be lost in stages, giving ions such as Fe²⁺ and Fe³⁺
- Catalytic activity: transition metals and their compounds act as catalysts (e.g. Fe in the Haber process, V₂O₅ in the Contact process) because they can change oxidation state during a reaction and provide a surface for reactant adsorption
- Coloured compounds: partially filled d-orbitals allow d-d electronic transitions that absorb specific wavelengths of visible light
- Formation of complex ions: transition metal cations can form covalent coordination bonds with ligands
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