Question

Compare the lanthanoid and actinoid elements with reference to the following: (i) Atomic and ionic sizes (ii) Oxidation state. (1½+1½=3)

Hint:

Both show a steady f-contraction (lanthanoid vs actinoid contraction, larger for actinoids). Lanthanoids are mostly +3; actinoids show many oxidation states because 5f, 6d and 7s are close in energy.

Answer 1

Concept: Lanthanoids are the 4f-series (Ce to Lu) and actinoids are the 5f-series (Th to Lr). Both are inner-transition (f-block) elements, but they differ because 5f electrons are more diffuse and shield the nucleus less effectively than 4f electrons.

Step 1: (i) Atomic and ionic sizes.
In both series the atomic and ionic radii decrease steadily as we move across, because each added f-electron shields the nuclear charge poorly, so the effective nuclear charge felt by the outer electrons rises and pulls them in.
• In lanthanoids this gradual decrease is called the lanthanoid contraction.
• In actinoids the same type of decrease is called the actinoid contraction. Because 5f electrons shield even more poorly than 4f electrons, the contraction from element to element is slightly greater in the actinoids.

Step 2: (ii) Oxidation state.
• Lanthanoids show mainly the +3 oxidation state as their common and stable state; a few also show +2 or +4 (for example Eu²⁺, Ce⁴⁺) but these are less common.
• Actinoids show a much wider range of oxidation states (from +3 up to +7, for example U shows +3, +4, +5 and +6). This is because in actinoids the 5f, 6d and 7s orbitals lie close in energy, so more electrons can take part in bonding.

Result: Both series show a steady contraction in size (a little larger in actinoids), but lanthanoids are dominated by the +3 state whereas actinoids display many oxidation states owing to the comparable energies of their 5f, 6d and 7s orbitals.