Periodic Table

(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 .
However, and oxidation states are also shown in some compounds (e.g., Eu , Ce ) due to stability of half-filled or empty f-orbitals.
\underline{Actinoids:}
Actinoids exhibit a much wider range of oxidation states, from to and beyond.
For example, U shows , while Np and Pu show multiple states.
This is because 5f, 6d, and 7s orbitals have comparable energies and all can participate in bonding.

Lanthanides, which consist of the 14 elements from atomic numbers 58 (Cerium) to 71 (Lutetium), exhibit a wide range of oxidation states. These oxidation states arise from the loss of electrons from the 4f, 5s, and 5p orbitals. The oxidation states of lanthanides are generally determined by the availability of the 4f electrons for bonding. Common Oxidation States: 1. +3 Oxidation State (Most Common):
The most common and stable oxidation state for lanthanides is +3. In this state, the three 4f electrons are lost, leaving the lanthanide in a stable, highly charged state. This is the predominant oxidation state for the majority of the lanthanides. For example, the most common ion in lanthanide chemistry is , , and . 2. +2 Oxidation State:
Some lanthanides like europium (Eu) and ytterbium (Yb) can also exist in the +2 oxidation state. The +2 state is observed when the lanthanide loses two 4f electrons. The lanthanide ions in this state tend to be more stable in an environment that provides additional stabilization, such as when they are complexed with ligands. 3. +4 Oxidation State:
A few lanthanides, such as cerium (Ce) and thorium (Th), can exhibit the +4 oxidation state. In this state, they lose all of their 4f electrons and can bond with more electronegative species such as oxygen or halides. The +4 state is less stable compared to the +3 state and is generally observed in rare cases, usually when the lanthanide is in a more oxidizing environment. Rare Oxidation States: - +5 and +6 Oxidation States: These are very rare and unstable oxidation states for lanthanides. Some lanthanides like praseodymium (Pr) and neodymium (Nd) can exhibit the +5 oxidation state under special conditions, but they are rarely encountered in nature. Summary:
- Lanthanides predominantly exhibit the +3 oxidation state.
- The +2 and +4 oxidation states are observed in some lanthanides but are less stable.
- The +5 and +6 oxidation states are very rare and highly unstable.