The electronic configuration of Fe2+ and Mn2+ ions are as follows:
Fe2+ is [Ar]18 3d6.
Mn2+ is [Ar]18 3d5. As it is known the half and completely filled orbitals have more stability. Hence, Mn with (+2) state has a stable d5 half- filled electronic configuration due to which Mn2+ resists to oxidise to Mn3+. We can find that, Fe2+ has 3d6 configuration and by losing one electron, its configuration can change to a more stable 3d5 half- filled electronic configuration. Therefore, Fe2+ easily gets oxidized to Fe+3 oxidation state.