F-Block Elements: Lanthanoids & Actinoids
The F-block elements, also known as inner transition elements, consist of the Lanthanoids ($4f$) and Actinoids ($5f$). Their unique magnetic properties, color, and contraction effects make them a critical part of Inorganic Chemistry for JEE and NEET.
1. Electronic Configuration & Stability
The general electronic configuration of f-block elements is:
Key Stability Trends
- Common Oxidation State: $+3$ is the most stable state for almost all Lanthanoids.
- Anomalous States:
- $Ce^{+4} (4f^0)$ and $Tb^{+4} (4f^7)$ form stable ions (act as oxidizing agents).
- $Eu^{+2} (4f^7)$ and $Yb^{+2} (4f^{14})$ are stable (act as reducing agents).
Figure: Comprehensive Chart of Lanthanoid Properties and Reactions
2. Lanthanoid Contraction
The steady decrease in atomic and ionic radii of Lanthanoids with increasing atomic number is called Lanthanoid Contraction.
Causes & Consequences
- Cause: Poor shielding effect of $4f$ electrons. The nuclear charge increases by +1 at each step, but the 4f electrons do not shield the outer electrons effectively, pulling the shell closer.
- Similarity in Size (4d & 5d): Elements like Zirconium-Hafnium ($Zr-Hf$) and Niobium-Tantalum ($Nb-Ta$) have almost identical radii.
- Basicity Trend: As size decreases from $La^{3+}$ to $Lu^{3+}$, the covalent character of hydroxides increases. Thus, $La(OH)_3$ is most basic and $Lu(OH)_3$ is least basic.
3. Actinoids & Comparison
Actinoids involve the filling of $5f$ orbitals. Unlike Lanthanoids, they show a wider range of oxidation states ($+3, +4, +5, +6, +7$) because the energy gap between $5f, 6d,$ and $7s$ orbitals is very small.
Key Differences
- Contraction: Actinoid contraction is greater than Lanthanoid contraction because $5f$ orbitals have even poorer shielding than $4f$ orbitals.
- Radioactivity: All Actinoids are radioactive.
- Chemical Reactivity: Actinoids are generally more reactive, especially in fine powder form, reacting rapidly with boiling water, acids, and non-metals.
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