Explain trends in `M^(2+)//M` standard electrode potentials

Explanation

The stability of `M^(2+)` ions in aqueous medium depends on three factors: (i) Enthalpy of atomisation (ii) Summation of first and second ionization enthalpies (iii) Hydration enthalpy
An elements in `Mn^(2+)` state in aqueous medium is more stabler if the electrode potential `(M^(2+)//M)` value of more negative. Across the period, the tendency to form `M^(2+)` ion decreases.
Except copper, all elements of first transition series show negative values of electrode potentials. The exceptional behaviour of the copper due to low 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 positive electrode potential, copper doesnot 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. However, the electrode potential values of Mn and Zn are lowered because of low second ionization enthalpies while Ni has exceptionally more negative electrode potential due to its high hydration enthalpy.

Across the period `M^(2+)//M` value decreases because of increase in first and second ionization enthalpies