f-Block Elements
The Inner Transition Elements: Lanthanoids and Actinoids.
The f-block elements are those in which the differentiating electron enters the $(n-2)f$ orbitals. They are known as Inner Transition Elements and consist of two series: Lanthanoids (4f) and Actinoids (5f).
1. Electronic Configuration
General Formula
Lanthanoids (4f Series)
Ce (Z=58) to Lu (Z=71).
General config: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.
Actinoids (5f Series)
Th (Z=90) to Lr (Z=103).
General config: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$.
2. The Lanthanoids
Key Characteristics
1. Oxidation States:
The most common oxidation state is +3. However, +2 and +4 are also exhibited to attain stable configurations ($f^0, f^7, f^{14}$).
- Europium (Eu): Shows +2 ($[Xe]4f^7$) but is a strong reducing agent changing to +3.
- Cerium (Ce): Shows +4 ($[Xe]4f^0$) but is a strong oxidizing agent changing to +3.
2. Color and Magnetism:
Many trivalent lanthanoid ions are colored due to f-f transitions. They are paramagnetic due to unpaired f-electrons.
3. Lanthanoid Contraction
Cause and Consequences
Cause: The steady decrease in atomic and ionic radii from La to Lu is called Lanthanoid Contraction. It occurs because the 4f electrons provide poor shielding against the increasing nuclear charge.
- Similarity in Radii: The atomic radii of elements of the second (4d) and third (5d) transition series are almost similar (e.g., Zr $\approx$ Hf).
- Difficulty in Separation: Due to similar chemical properties (similar size and charge), lanthanoids are difficult to separate.
- Basicity Difference: As size decreases from La to Lu, the covalent character of hydroxides increases. Thus, basic strength decreases ($La(OH)_3$ is most basic, $Lu(OH)_3$ is least).
4. The Actinoids
Radioactive Elements
- Oxidation States: Actinoids show a greater range of oxidation states (+3, +4, +5, +6, +7) compared to lanthanoids. This is because the energy energies of 5f, 6d, and 7s are comparable.
- Actinoid Contraction: Similar to lanthanoid contraction but greater in magnitude due to even poorer shielding by 5f electrons compared to 4f.
- Reactivity: They are highly reactive metals, especially when finely divided.
5. Comparison: Lanthanoids vs Actinoids
| Property | Lanthanoids | Actinoids |
|---|---|---|
| Binding Energy | 4f electrons have higher binding energy. | 5f electrons have lower binding energy. |
| Oxidation States | Show +2, +3, +4 (+3 is dominant). | Show variable states up to +7. |
| Complex Formation | Less tendency to form complexes. | Greater tendency to form complexes. |
| Radioactivity | Non-radioactive (except Promethium). | All are radioactive. |
| Oxo-cations | Do not form oxo-cations. | Form oxo-cations (e.g., $UO_2^{2+}$). |
6. Applications
- Mischmetal: An alloy of Lanthanoid metal (~95%) and Iron (~5%) with traces of S, C, Ca, Al. Used in magnesium-based alloy to produce bullets, shells, and lighter flints.
- Nuclear Fuel: Uranium and Plutonium are used as nuclear fuel.
- Catalysts: Cerium salts are used in quantitative analysis.
Knowledge Check
Test your understanding of f-Block Elements
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