Which among the following lanthanoids forms the most stable $+2$ oxidation state?

Which of the following elements belongs to the actinoid series?

State the consequences of lanthanoid contraction. Explain the formation of coloured ions by lanthanoids.

Which element from the following has a half-filled $4f$ orbital in its observed ground state?

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.

Assertion : Ionisation enthalpies of the early actinoids are lower than for early lanthanoids.
Reason : $5f$ electrons are more effectively shielded from nuclear charge than $4f$ electrons.