Identify the non-radioactive element from the following.

Identify the isoelectronic pair of ions from the following.

Name a member of the lanthanoid series which is well known to exhibit $+4$ oxidation state.

Although $(+3)$ oxidation state is the characteristic oxidation state of lanthanoids,cerium shows $(+4)$ oxidation state also. Why?

The $f$ orbitals are half-filled and completely filled,respectively,in which of the following lanthanide ions?
(Given: Atomic numbers: $Eu=63, Sm=62, Tm=69, Tb=65, Yb=70, Dy=66$)

The element having electronic configuration $ns^2(n-1)d^{1-10}(n-2)f^{1-14}$ belongs to: