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

Which orbitals,$4f$ or $5f$,are more capable of participating in bond formation?

If $Ln$ represents a lanthanoid element,find the general formula of lanthanoid hydroxide.

Fill in the blanks:
$1.$ The atomic radii of the elements of the third transition series are .......... to those of the corresponding elements of the second transition series.
$2.$ Samarium melts at .......... $K$.
$3.$ The color of lanthanoid ions is due to the presence of .......... electrons.

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

$4f^2$ electronic configuration is found in