The incorrect statement among the following is:

Solution & Explanation

\textbf{Answer:} (d) Most of the trivalent lanthanoid ions are colourless in the solid state. (This is the incorrect statement.) \textbf{Solution:} (a) Lanthanoids are metals and are good conductors of heat and electricity — correct. (b) Actinoids are highly reactive metals, especially in a finely divided state — correct. (c) The element-to-element contraction is greater in actinoids than in lanthanoids because the $5\mathrm{f}$ electrons shield the nuclear charge poorly — correct. (d) MOST trivalent lanthanoid ions ($\mathrm{Ln}{}^{3+}$) are actually coloured (due to f-f transitions); only a few such as $\mathrm{La}{}^{3+}$ and $\mathrm{Lu}{}^{3+}$ are colourless. So the statement that most are colourless is incorrect. Hence the incorrect statement is (d).