$Ln$ (Lanthanide) $\xrightarrow{\text{With Halogen}} X$
$Ln$ (Lanthanide) $\xrightarrow{\text{Burn With } O_2} Y$
$Ln$ (Lanthanide) $\xrightarrow{\text{Heated with } N_2} Z$
$X, Y$ and $Z$ are respectively:-

Which of the following statements is incorrect?

Which is the general formula for the electronic configuration of the actinide series?

Assertion : Europium $(II)$ is more stable than cerium $(II)$.
Reason : Cerium salts are used as catalyst in petroleum cracking.

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 ......... .

Explain the lanthanoid contraction.