Question #44297

In aqueous acidic solution at $298\ \text{K}$, Plutonium ($\ce{Pu}$) exhibits a unique chemical behavior where four oxidation states ($+3$, $+4$, $+5$, and $+6$) can coexist in equilibrium. This multiplicity of accessible oxidation states is fundamentally linked to the anomalously small energy gaps between its successive ionization energies ($IE_3$, $IE_4$, $IE_5$, $IE_6$) compared to its Lanthanoid congener, Samarium ($\ce{Sm}$).\n\nWhich of the following combinations of quantum mechanical phenomena correctly explains this deviation from standard periodic trends?\n\n\textbf{I.} Direct relativistic effects cause the radial contraction and energetic stabilization of the $s$ and $p$ orbitals due to the relativistic mass increase of electrons near the highly charged nucleus.\n\textbf{II.} Indirect relativistic effects result in the radial expansion and energetic destabilization of the $5f$ and $6d$ orbitals due to enhanced screening of the nuclear charge by the contracted inner $s$ and $p$ orbitals.\n\textbf{III.} The relativistic contraction of the $7s$ orbital induces a strong inert pair effect, which paradoxically lowers the energy required to remove the $5f$ electrons, thereby decreasing $IE_3$ and $IE_4$.\n\textbf{IV.} Unlike the deeply buried $4f$ orbitals in Lanthanoids, the $5f$ orbitals in early Actinoids are spatially extended, allowing them to participate extensively in covalent bonding (e.g., in $\ce{PuO2^2+}$).