The electrode potential of M²⁺/M of 3d  series elements shows positive value for :

Only copper has a positive electrode potential of M²⁺/M in the 3d^-series elements.

${\mathrm{E}}^{⊝}/{\mathrm{V}}_{\left({\mathrm{Cu}}^{+2}/\mathrm{Cu}\right)}: +0.34\mathrm{V}$