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.

(N/A) $1.$ The atomic radii of the elements of the third transition series are almost identical to those of the corresponding elements of the second transition series due to lanthanoid contraction.
$2.$ Samarium melts at $1347 \ K$.
$3.$ The color of lanthanoid ions is due to the presence of $f$-electrons (specifically $f-f$ transitions).