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Q4.33(iii):
Compare the chemistry of the actinoids with that of lanthanoids with reference to: (iii) chemical reactivity.

Solution :

Step 1: Theoretical Foundation of Reactivity

The chemical reactivity of lanthanoids ($Ln$) and actinoids ($An$) is governed by the ionization enthalpy and the shielding effectiveness of the inner subshells. Both series are highly electropositive and reactive metals. However, the reactivity differences are dictated by the energy distribution of the $4f$ and $5f$ orbitals.

Step 2: Comparative Analysis of Chemical Reactivity

The reactivity is categorized based on the trend in electropositivity and the nature of the chemical bonds formed.

Feature Lanthanoids ($4f$) Actinoids ($5f$)
General Reactivity Highly reactive, similar to calcium. More reactive than lanthanoids due to lower ionization energy.
Electrode Potential Generally high negative standard electrode potentials ($E^\circ$). More negative $E^\circ$ values compared to lanthanoids, indicating higher reducing power.
Reaction with Non-metals React with $N_2, H_2, C,$ and $S$ under specific conditions. React more readily with non-metals [Per the high spatial extent of $5f$ orbitals].
Oxidation States Predominantly $+3$. Variable oxidation states ($+3, +4, +5, +6, +7$).

Step 3: Justification of Observations

  • Ionization Energy: The $5f$ electrons are less effectively shielded than $4f$ electrons [Per the Slater's Rules Application]. Consequently, the ionization enthalpy of actinoids is generally lower than that of lanthanoids, making them more electropositive and chemically reactive.
  • Oxidation State Variability: The energy gap between $5f, 6d,$ and $7s$ orbitals in actinoids is small. This allows for multi-valent behavior and complex redox chemistry which is less pronounced in lanthanoids [Per the Principle of Degeneracy and Orbital Energy Levels].
  • Kinetics: Because actinoids have a higher tendency to form complex ions and involve $d$-orbital participation in covalent bonding, their kinetic behavior in solution is more diverse than the predominantly ionic nature of lanthanoid chemistry.

Step 4: Visualizing the Reactivity Gradient

Atomic Number (Z) Reactivity (Reducing Power) Lanthanoids Actinoids

Final Solution: The actinoids exhibit higher chemical reactivity compared to the lanthanoids. This is primarily attributed to the lower ionization energy of actinoids caused by the poorer shielding effect of the $5f$ electrons, facilitating more versatile oxidation states and stronger reducing power across the series.


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