Explain the trends in $M^{2+}/M$ standard electrode potentials.

(N/A) The stability of $M^{2+}$ ions in an aqueous medium depends on three factors:
$(i)$ Enthalpy of atomisation
$(ii)$ Summation of first and second ionization enthalpies
$(iii)$ Hydration enthalpy
An element in the $M^{2+}$ state in an aqueous medium is more stable if the electrode potential $(M^{2+}/M)$ value is more negative. Across the period,the tendency to form $M^{2+}$ ions generally decreases.
Except for copper,all elements of the first transition series show negative values of electrode potentials. The exceptional behavior of copper is due to its high enthalpy of atomisation and very high summation of first and second ionization enthalpies,which is not compensated by its hydration enthalpy $(Cu^{2+})$.
Because of its positive electrode potential,copper does not liberate hydrogen gas from dilute acids and reacts only with oxidizing acids such as nitric acid and hot concentrated sulphuric acid.
The electrode potentials of $Mn$,$Ni$,and $Zn$ are more negative than expected. The electrode potential values of $Mn$ and $Zn$ are lowered because of their stable electronic configurations (half-filled $d^5$ for $Mn^{2+}$ and fully-filled $d^{10}$ for $Zn^{2+}$),while $Ni$ has an exceptionally more negative electrode potential due to its high hydration enthalpy.