Why are the $E^{\circ}$ values for $Mn^{2+}$,$Ni^{2+}$,and $Zn^{2+}$ more negative than expected?

What is the general electronic configuration of the outermost shell of $f$-block elements?

The general electronic configuration of transition elements is

Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:
$(i)$ electronic configurations,
$(ii)$ oxidation states,
$(iii)$ ionisation enthalpies,and
$(iv)$ atomic sizes.

Assertion : Transition metals show variable valency.
Reason : Transition metals have a large energy difference between the $ns^2$ and $(n-1)d$ electrons.

Transition elements exhibit variable oxidation states because they lose electrons from which of the following orbitals?