Name the elements of the 3d transition series which shows the maximum number of oxidation states. Why does it show so?

(i) Manganese (3d⁵ 4s²) shows the maximum number of oxidation states as its atoms have unpaired electrons in 3d orbitals. It shows all oxidation states from +2 to +7.
(ii) Cu has a positive E⁰(M²⁺/M) value, as the high energy (Δa⁺ + ΔiH) to transform Cu(s) to Cu²⁺(aq) is not balanced by hydration enthalpy.
(iii) Mn³⁺ is a stronger oxidizing agent as the charge from Mn³⁺ to Mn²⁺ results in a half-filled, d⁵ configuration which has extra stability.
(iv) Europium, (Eu²⁺) is formed by losing two electrons and its (Xe)f⁷ configuration accounts for its formation.
(v) MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O