Definition of a Transition Element
A transition element is a d-block element that forms at least one stable ion with an incomplete d-subshell (\(\text{d}^1\) to \(\text{d}^9\)).
Key Exclusions: \(\text{Sc}\) forms only \(\text{Sc}^{3+}\) (\(\text{d}^0\)) and \(\text{Zn}\) forms only \(\text{Zn}^{2+}\) (\(\text{d}^{10}\)); neither is a transition element under this definition.
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 the 4s subshell fills before 3d (Aufbau principle), when transition metals ionise, the 4s electrons are always lost first. Once the 3d orbitals are occupied, 3d electrons repel the 4s electrons outward, making 4s higher in energy.
Example: \(\text{Fe} = [\text{Ar}]\,3\text{d}^6\,4\text{s}^2 ightarrow \text{Fe}^{2+} = [\text{Ar}]\,3\text{d}^6 ightarrow \text{Fe}^{3+} = [\text{Ar}]\,3\text{d}^5\)
Anomalous Configurations: Chromium and Copper
Chromium and copper exhibit anomalous ground-state electron configurations due to minimized electron repulsion and symmetrical charge distribution:
- Cr (Z = 24): \([\text{Ar}]\,4\text{s}^1\,3\text{d}^5\) (half-filled d-subshell, not \(4\text{s}^2\,3\text{d}^4\))
- Cu (Z = 29): \([\text{Ar}]\,4\text{s}^1\,3\text{d}^{10}\) (fully filled d-subshell, not \(4\text{s}^2\,3\text{d}^9\))
Absorption vs Emission in Coloured Complexes
Do not write "colour is emitted when electrons fall down" for transition metal complexes. In complex ions, visible light is absorbed when a d-electron is promoted between split d-orbitals (\(\text{d}\text{--}\text{d}\) transition). The colour transmitted/observed is the complementary colour to the absorbed wavelengths.
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