Which one of the following statements related to lanthanons is incorrect?

Solution & Explanation

\textbf{Answer:} (c) All the lanthanons are much more reactive than aluminium. (This is the incorrect statement.) \textbf{Solution:} (a) Eu attains a stable half-filled $4\mathrm{f}{}^7$ in the $+2$ state, so it does show $+2$ — correct. (b) Due to the lanthanoid contraction the ionic radius falls from Pr to Lu, so the hydroxides become less basic — correct. (c) Lanthanoids are reactive metals, but they are NOT "much more reactive than aluminium" in the way stated; their reactivity is comparable to (and the statement over-generalises against) the active metals — this is the incorrect statement. (d) $\mathrm{Ce}{}^{4+}$ ($\mathrm{Ce}(\mathrm{IV})$) is a well-known oxidising agent used in volumetric (cerimetric) analysis — correct. Hence the incorrect statement is (c).