Q.Compare lanthanides and actinides.

Six systematic contrasts distinguish the two inner-transition series. (1) Differentiating electron: enters 4f (lanthanoids) vs 5f (actinoids). (2) Binding energy of that f sub-shell is HIGHER for 4f (lanthanoids) but LOWER for 5f (actinoids) -- so actinoid f electrons are more chemically available. (3) Complex formation: lanthanoids show LESS tendency (tightly-bound, largely inert 4f electrons); actinoids show GREATER tendency (more available 5f + 6d electrons). (4) Colour: most lanthanoids are colourless (Laporte-forbidden 4f-4f transitions, weak absorption); most actinoids ARE coloured -- e.g. U³⁺ red, U⁴⁺ green, UO₂²⁺ yellow, the same element showing three different colours across three oxidation states. (5) Oxo-cations: lanthanoids do NOT form …