Lanthanides and actinides resemble each other in:

Electron configuration

The correct answer is option 4. Electron configuration.

Let us explore in more detail why lanthanides and actinides resemble each other primarily in terms of their electron configuration:

Lanthanides :

Lanthanides are characterized by the filling of \(4f\) orbitals. The general electron configuration for lanthanides is \([Xe] 4f^{1-14} 5d^{0-1} 6s^2\). The 4f orbitals are gradually filled across the lanthanide series as you move from cerium (Ce) to lutetium (Lu).

Actinides:

Actinides, on the other hand, have their valence electrons filling the \(5f\) orbitals. The general electron configuration for actinides is \([Rn] 5f^{0-14} 6d^{0-1} 7s^2\). The 5f orbitals are gradually filled across the actinide series from actinium (Ac) to lawrencium (Lr).

Resemblance in Electron Configuration:

Similar Outer Electron Configurations: Both lanthanides and actinides have similar outer electron configurations due to the sequential filling of either the 4f or 5f orbitals. This similarity leads to comparable chemical properties in terms of their ability to form complexes and exhibit various oxidation states.

Chemical Behavior: The partially filled \(4f\) and 5f orbitals in lanthanides and actinides, respectively, contribute to their ability to form stable complexes with ligands and exhibit multiple oxidation states. This versatility is a key characteristic shared by both series.

Differences and Similarities in Other Properties:

Ionization Energy: Lanthanides and actinides generally have lower ionization energies compared to transition metals due to the shielding effect of inner electrons. However, there can be variations in ionization energies within each series.

Oxidation States: Both series can exhibit a wide range of oxidation states, but the preferred oxidation states and chemical reactivity can vary. Lanthanides often exhibit +3 oxidation states prominently, while actinides can exhibit a wider range including higher oxidation states due to the greater accessibility of \(5f\) orbitals.

Conclusion:

The primary similarity between lanthanides and actinides lies in their electron configurations, specifically the filling of the \(4f\) and \(5f\) orbitals, respectively. This similarity influences their chemical behavior, particularly their ability to form complexes and exhibit various oxidation states. While they share these similarities, differences in ionization energy and oxidation state preferences also exist between the two series.

Therefore, when considering what lanthanides and actinides resemble each other in, electron configuration stands out as the most significant factor shaping their chemical properties and behaviors.