Which of the following hydroxides is the most basic?

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

Explain the lanthanoid contraction.

Which one of the following statements concerning lanthanide elements is false?

$Ce$ $(Z=58)$ is a member of

Fill in the blanks:
$1.$ In the elements of the actinoid series,the last electron enters the ........... orbitals.
$2.$ Inner transition elements are the elements of the .......... series.
$3.$ If electrons are filled in the $5f$ series,those elements are called ....... elements.
$4.$ Many elements of the actinoid series are ......... .