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Q4.33(ii):
Compare the chemistry of the actinoids with that of lanthanoids with reference to: (ii) oxidation states

Solution :

Step 1: Theoretical Foundation of Oxidation States

The electronic configuration of lanthanoids is represented as $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$, while actinoids are represented as $[Rn] 5f^{0-14} 6d^{0-1} 7s^2$. The primary distinction in oxidation state diversity arises from the spatial extent and shielding efficacy of the $f$-orbitals [Per the principles of Quantum Mechanics and Slater’s Rules].

Step 2: Analysis of Lanthanoids

Lanthanoids exhibit a limited range of oxidation states because the $4f$ electrons are deeply buried within the atom and are not readily available for bonding [Justification: Strong core-like character of $4f$ orbitals].

  • Common Oxidation State: $+3$ is the predominant state for all lanthanoids.
  • Anomalous States: States such as $+2$ and $+4$ are occasionally observed only when they lead to stable configurations, such as empty ($f^0$), half-filled ($f^7$), or fully-filled ($f^{14}$) subshells [e.g., $Ce^{4+}$ ($4f^0$), $Eu^{2+}$ ($4f^7$), $Tb^{4+}$ ($4f^7$), $Yb^{2+}$ ($4f^{14}$)].

Step 3: Analysis of Actinoids

Actinoids exhibit a much broader range of oxidation states compared to lanthanoids. This is attributed to the fact that the $5f$, $6d$, and $7s$ energy levels are comparable in energy [Justification: Decreased inter-electronic repulsion and reduced energy gap between $5f$ and $6d$ subshells].

  • Diversity: Actinoids display oxidation states such as $+3, +4, +5, +6,$ and sometimes $+7$.
  • Transition to Variability: Early actinoids (like $Th, Pa, U, Np, Pu$) show a greater variety of oxidation states because their $5f$ electrons are less effectively shielded than $4f$ electrons, allowing them to participate in covalent bond formation.

Step 4: Comparative Data Summary

Parameter Lanthanoids ($4f$) Actinoids ($5f$)
Primary State $+3$ $+3$ (common for later elements)
Range Limited ($+2, +3, +4$) Broad ($+3$ to $+7$)
Energy Gap High ($4f$ electrons are inert) Low ($5f, 6d, 7s$ are comparable)

Step 5: Visual Representation of Orbital Involvement

Lanthanoid ($4f$ shielded) Actinoid ($5f$ available) +3 (Primary) +3, +4, +5, +6

Final Solution: Lanthanoids exhibit a limited range of oxidation states, primarily $+3$, due to the stable, buried nature of $4f$ orbitals. Conversely, actinoids show a wide range of oxidation states ($+3$ to $+7$) because the $5f, 6d,$ and $7s$ energy levels are energetically comparable, facilitating easier participation of $5f$ electrons in chemical bonding.


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