Question

Knowing that the chemistry of lanthanoids (Ln) is dominated by its $+3$ oxidation state, which of the following statements is incorrect?

• A. The ionic sizes of Ln(III) decrease in general with increasing atomic number.
• B. Ln(III) compounds are generally colourless
• C. Ln(III) hydroxides are mainly basic in character.
• D. None of the above

Answer 1

The correct option is B Ln(III) compounds are generally colourless

In lanthanide series, atomic and ionic radii of elements decreases with an increase in atomic number fro La to Lu. This is due to imperfect shielding of one electron by another in the same f -subshell which is the cause for lanthanoid contraction.

Most of the Ln(III) compounds are coloured in solid as well as in aqueous phase due to incomplete filling of f-orbitals except $L{a}^{+3}$ and $L{u}^{+3}$.

Oxides and hydroxides of lanthanide $\left(L{n}_2{O}_3\right)$ are basic in nature. Across the lanthanide series the size of the cations decreases thereby decreasing ionic character and increasing covalent character of hydroxides of lanthanides. Thus, the basicity of hydroxides decreases.

Hence, statement (b) is incorrect.