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Q4.20(iv):
Compare the chemistry of actinoids with that of the lanthanoids with special reference to: (iv) chemical reactivity.

Solution :

Step 1: Theoretical Foundation of Chemical Reactivity

The chemical reactivity of $f$-block elements is governed by the ionization enthalpy, the energy associated with the removal of valence electrons, and the shielding efficiency of the $f$-orbitals. Lanthanoids $(4f)$ and actinoids $(5f)$ exhibit distinct trends due to the spatial extension and energy levels of their respective subshells.

Step 2: Comparative Analysis of Reactivity

The chemical reactivity is evaluated based on the following criteria:

Feature Lanthanoids Actinoids
General Reactivity High reactivity, similar to alkaline earth metals. Highly reactive, significantly more so than lanthanoids.
Oxidation States Predominantly $+3$. Variable ($+3, +4, +5, +6, +7$).
Electronegativity Generally lower, exhibiting more metallic behavior. Higher, implying greater covalent character in bonds.

Step 3: Justification of Higher Reactivity in Actinoids

  • Orbital Shielding [Per Screening Effect Theory]: The $5f$ orbitals in actinoids are less shielded than the $4f$ orbitals in lanthanoids. Consequently, the valence electrons in actinoids are more effectively attracted by the nucleus, yet they are more readily available for chemical bonding due to the smaller energy gap between $5f, 6d,$ and $7s$ orbitals.
  • Ionization Enthalpy [Per Periodic Trends]: Actinoids exhibit lower first ionization enthalpies compared to lanthanoids. This facilitates the loss of electrons during chemical reactions, rendering them chemically more active.
  • Oxidation State Variety [Per Thermodynamic Stability]: Unlike lanthanoids which are restricted to the $+3$ state due to high stability, actinoids possess a range of oxidation states. This allows them to participate in a broader spectrum of chemical interactions, including the formation of complex oxocations like $UO_2^{2+}$ and $PuO_2^{2+}$.

Visual Representation of Reactivity Trends

Lanthanoids Actinoids Reactivity Level

Step 4: Conclusion on Chemical Behavior

The actinoids are chemically more reactive than lanthanoids because the $5f$ electrons are not as deeply buried as $4f$ electrons. This structural arrangement, combined with the smaller energy difference between $5f, 6d,$ and $7s$ energy levels, allows actinoids to form a wider range of compounds and exhibit greater sensitivity to chemical reagents, such as boiling water or acids.

Final Solution: Actinoids are more reactive than lanthanoids due to lower ionization enthalpies and the ability to utilize $5f$ electrons in bonding, resulting in a broader range of oxidation states and higher chemical versatility.


More Questions from Class 12 Chemistry The d- and f-Block Elements Exercises


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