Question

Explain the following:
(i) Lanthanoid contraction
(ii) Oxidation states in Lanthanoids and Actinoids

Hint:

Lanthanoids: mostly \(+3\).
Actinoids: variable oxidation states due to availability of 5f, 6d, and 7s electrons.

Answer 1

(i) Lanthanoid Contraction:
As we move from La (Z=57) to Lu (Z=71) in the lanthanoid series, the electrons are added to the 4f orbitals.
The 4f electrons have very poor shielding effect, so the increasing nuclear charge pulls the outer 5d and 6s orbitals closer to the nucleus.
This results in a steady decrease in atomic and ionic radii, known as lanthanoid contraction.
Consequences:
- Similarity of size between elements of second and third transition series.
- Difficulty in separation of lanthanoids.
- Greater basicity differences in lanthanoid hydroxides.
--- (ii) Oxidation States in Lanthanoids and Actinoids:
\underline{Lanthanoids:}
The most common oxidation state is \(+3\).
However, \(+2\) and \(+4\) oxidation states are also shown in some compounds (e.g., Eu\(^{2+}\), Ce\(^{4+}\)) due to stability of half-filled or empty f-orbitals.
\underline{Actinoids:}
Actinoids exhibit a much wider range of oxidation states, from \(+3\) to \(+6\) and beyond.
For example, U shows \(+3, +4, +5, +6\), while Np and Pu show multiple states.
This is because 5f, 6d, and 7s orbitals have comparable energies and all can participate in bonding.