Identify the formula of compound $(B)$ formed in the following reaction:
$(i)$ Lanthanoids $(Ln)$ are burnt in oxygen to form compound $(A)$.
$(ii)$ Compound $(A)$ reacts with $CO_2$ in excess to form compound $(B)$.

If the electronic configuration of $M^{3+}$ is $[Xe] \ 4f^3$,then $M^{3+}$ is

Which of the following elements have half-filled $f$-orbitals in their ground state?
(Given: atomic number $Sm = 62$; $Eu = 63$; $Tb = 65$; $Gd = 64$; $Pm = 61$)
$A.$ $Sm$ $B.$ $Eu$ $C.$ $Tb$ $D.$ $Gd$ $E.$ $Pm$
Choose the correct answer from the options given below:

Which oxidation state is common for lanthanoid elements?

Lanthanide contraction occurs because

The $Eu^{2+}$ ion is a strong reducing agent in spite of its ground state electronic configuration (outermost) : [Atomic number of $Eu=63$]