Lanthanoids and actinoids

The essentials

The f-block has two series. Lanthanoids (Ce to Lu, after lanthanum) fill the 4f orbitals; actinoids (Th to Lr, after actinium) fill the 5f orbitals. They are sometimes called inner transition elements.

Key points

• Lanthanoids mainly show the +3 oxidation state. Ce⁴⁺ (stable f⁰) is a good oxidising agent; Eu²⁺ and Yb²⁺ act as reducing agents.
• Lanthanoid contraction: a steady decrease in atomic and ionic radii from La to Lu, because 4f electrons shield the increasing nuclear charge poorly.
• Consequences: elements of the second and third transition series in the same group have nearly equal radii (Zr 160 pm, Hf 159 pm), so Zr and Hf are very similar and occur together; basicity of Ln(OH)₃ decreases from La(OH)₃ to Lu(OH)₃.
• Lanthanoids are reactive metals; they react with water to liberate hydrogen and burn in oxygen to form Ln₂O₃.
• Actinoids are all radioactive. They show a wider range of oxidation states (for example up to +7 for Np and Pu) because 5f, 6d and 7s levels have comparable energies.
• Actinoid contraction is greater from element to element than lanthanoid contraction because 5f electrons shield even more poorly.
• Actinoid chemistry is harder to study because many are radioactive with short half-lives and are available only in small amounts.

Exam tips

• Remember: lanthanoids mostly +3; actinoids show a range of states, with +3 common but higher states important in the early members.

Common mistakes

• Saying lanthanoid contraction is caused by an increase in shielding; it is due to poor shielding.