The reason for lanthanoid contraction is:

Match the following:

$I. \ Zn^{2+}$	$(i) \ d^8 \ \text{configuration}$
$II. \ Cu^{2+}$	$(ii) \ \text{Colourless}$
$III. \ Ni^{2+}$	$(iii) \ \mu = 1.73 \ \text{BM}$

Which statement is true about the transition elements?

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ The atomic radii of the elements of the second transition series $(4d)$ are larger than those of the first transition series $(3d)$.
$(b)$ $Cr^{+}$,$Mn^{2+}$,and $Fe^{3+}$ have the same electronic configuration.
$(c)$ $Cr^{+}$,$Mn^{2+}$,and $Fe^{3+}$ have the same $d^5$ electronic configuration.

Calculate the number of unpaired electrons in the following gaseous ions: $Mn^{3+}, Cr^{3+}, V^{3+}$ and $Ti^{3+}$. Which one of these is the most stable in aqueous solution?

Which of the following triads has approximately equal size?