How would you account for the following situations?
With 3d⁴ configuration, Cr²⁺ acts as a reducing agent but Mn³⁺ acts as an oxidising agent. (Atomic masses, Cr = 24, Mn = 25).

It can be explained on the basis of E° values of Cr³⁺ | Cr²⁺ (–0.4 A)
Mn³⁺ | Mn²⁺(+ 1.5 V).

On the basis of these values, it is clear that Cr²⁺ has a greater tendency to oxidise into Cr³⁺, thus it acts as reducing agent.

Cr²⁺(aq) → Cr³⁺(aq) + e^–

While Mn³⁺ has a greater tendency to reduce into Mn²⁺, thus it acts as oxidising agent.
Mn³⁺(aq) + e^– → Mn²⁺(aq)