Why is Cr2+ reducing and Mn3+ oxidising when both have d4 configuration?

The oxidation state and the ability of an element to get oxidized or reduced depends on various factors such as electronegativity, ionization energy, and electron affinity.

1. Chromium (Cr) in Cr2+ state: Chromium has an atomic number of 24, and in its ground state, it has the electron configuration [Ar] 3d5 4s1. When it loses two electrons to form Cr2+, the electron configuration becomes [Ar] 3d4. This means it has four unpaired electrons in the 3d orbital. Chromium can further lose these electrons easily, which means it can be oxidized further. Therefore, it acts as a reducing agent (it gets oxidized).

2. Manganese (Mn) in Mn3+ state: Manganese has an atomic number of 25, and in its ground state, it has the electron configuration [Ar] 3d5 4s2. When it loses three electrons to form Mn3+, the electron configuration becomes [Ar] 3d4. This means it also has four unpaired electrons in the 3d orbital. However, Mn3+ has a high positive charge and a high oxidation state, which makes it difficult for it to lose more electrons. Instead, it tends to gain an electron to reduce the charge and become more stable. Therefore, it acts as an oxidizing agent (it gets reduced).

So, even though both Cr2+ and Mn3+ have a d4 configuration, their different oxidation states and the stability of these states determine whether they act as reducing or oxidizing agents.