The chemistry of the lanthanoids (Ln) is dominated by the +3 oxidatio…

The correct answer is Option (3) → Ln(III) compounds are generally colourless.

Most Ln³⁺ ions have partly filled 4f orbitals, and excitation of electrons within the f level makes them absorb visible light. So many trivalent lanthanoid ions are coloured, both in the solid state and in solution. Only the ions with an empty or a full 4f subshell, La³⁺ (4f⁰) and Lu³⁺ (4f¹⁴), are colourless.

Option (1) The lanthanoids form oxides of the formula Ln₂O₃. — Correct about the lanthanoids. In their common +3 state, two Ln³⁺ ions balance three O²⁻ ions, giving oxides M₂O₃.

Option (2) The ionic sizes of Ln(III) decrease in general with increasing atomic number. — Correct about the lanthanoids. This is the lanthanoid contraction: the 4f electrons shield one another poorly, so the rising nuclear charge pulls the electrons in.

Option (3) Ln(III) compounds are generally colourless. — Incorrect about the lanthanoids. Most Ln³⁺ ions are coloured; only La³⁺ and Lu³⁺ show no colour.

Option (4) Ln(III) hydroxides are basic in character. — Correct about the lanthanoids. The hydroxides M(OH)₃ are basic, like the alkaline earth metal hydroxides.

NCERT (2026-27 edition):
4.5.4 General Characteristics – “… Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of f electrons. Neither La³⁺ nor Lu³⁺ ion shows any colour but the rest do so. However, absorption bands are narrow, probably because of the excitation within f level.”