CHEMCA Free Chemistry Notes for JEE NEET Chemistry MadeEasy: f-Block Elements (Lanthanoids & 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

$$ (n-2)f^{1-14} (n-1)d^{0-1} ns^2 $$

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.

Consequences:

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.

f-Block Elements (Lanthanoids & Actinoids) | chemca

f-Block Elements