Q.Compare lanthanides and actinides.

Step 1. Differentiating electron: lanthanides fill the 4f orbital (following La); actinides fill the 5f orbital (following Ac).

Step 2. Radioactivity: lanthanides are (with trace exceptions like Pm) stable, non-radioactive elements; every single actinide is radioactive, and most have short half-lives, with only Th and U occurring naturally in significant amounts.

Step 3. f-orbital binding energy and complex formation: 4f orbitals have HIGHER binding energy (more tightly bound, less chemically available), so lanthanides show LESS tendency to form complexes; 5f orbitals have LOWER binding energy (more available), so actinides show GREATER tendency to form complexes.

Step 4. Colour: most lanthanides are colourless (Laporte-forbidden 4f-4f transitions); most actinides ARE coloured, e.g. U³⁺ red, U⁴⁺ green, UO₂²⁺ yellow.

Step 5. Oxo-cations: lanthanides do NOT form them; actinides DO, e.g. UO₂²⁺ (uranyl), NpO₂²⁺ (neptunyl). …

Work through the chapter's six-point systematic comparison -- filling orbital, radioactivity, orbital binding energy/complex-forming tenden …

• Omitting radioactivity as a point of comparison, since it is one of the most practically significant differences between the two series even though it is not one of the chapter's core six structural/chemical points. …