Find the atomic number of the element at which pairing in the $4f$ orbital starts.

Lanthanoid ions with $4f^7$ configuration are :
$A$. $Eu^{2+}$
$B$. $Gd^{3+}$
$C$. $Eu^{3+}$
$D$. $Tb^{3+}$
$E$. $Sm^{2+}$
Choose the correct answer from the options given below :

Which elements exhibit nearly identical properties?

What is the correct electronic configuration of $Th$ $(Z=90)$?

Name the members of the lanthanoid series which exhibit $+4$ oxidation state and those which exhibit $+2$ oxidation state. Try to correlate this type of behavior with the electronic configurations of these elements.

Which of the following statements are correct?
$I$. $P$ has the least negative electron gain enthalpy among $P, S, Cl$, and $F$.
$II$. In $Eu$ and $Yb$, lanthanoid contraction is not observed.
$III$. $Ce(OH)_3$ is the most basic among lanthanoid hydroxides.
$IV$. The radii of $Na$ and $Na^+$ are $95 \ pm$ and $186 \ pm$ respectively.