Identify the rare earth element from the following.

Provide the electronic configurations of lanthanoids $(4f)$ series elements $(Ln)$ and their ions $Ln^{2+}$ and $Ln^{3+}$,and state their characteristics.

Which oxidation state is common for lanthanoid elements?

Which element from the following is radioactive?

On the basis of Lanthanoid contraction,explain the following :
$(i)$ Nature of bonding in $La_2O_3$ and $Lu_2O_3$.
$(ii)$ Trends in the stability of oxo salts of lanthanoids from $La$ to $Lu$.
$(iii)$ Stability of the complexes of lanthanoids.
$(iv)$ Radii of $4d$ and $5d$-block elements.
$(v)$ Trends in acidic character of lanthanoid oxides.

The separation of $Lanthanoid$ elements is based on which of the following?