Europium is

Identify the correct decreasing order of ionic radii of the following lanthanoids: $Ce^{3+}$,$Pm^{3+}$,$Sm^{3+}$,and $Gd^{3+}$.

Which lanthanoid from the following exhibits $f^{14}$ configuration in the $+3$ oxidation state?

Which element from the following lanthanoids has a half-filled $f$-orbital in its observed and expected electronic configuration?

Which of the following elements shows the greatest similarity to the lanthanoids?

Compare the chemistry of actinoids with that of the lanthanoids with special reference to:
$(i)$ electronic configuration
$(ii)$ atomic and ionic sizes
$(iii)$ oxidation state
$(iv)$ chemical reactivity.