Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: In aqueous solutions $$Cr^{2+}$$ is reducing while $$Mn^{3+}$$ is oxidising in nature.
Reason R: Extra stability to half filled electronic configuration is observed than incompletely filled electronic configuration.
In the light of the above statement, choose the most appropriate answer from the options given below:

We evaluate the Assertion-Reason statement about $$Cr^{2+}$$ and $$Mn^{3+}$$.

Assertion A: In aqueous solutions, $$Cr^{2+}$$ is reducing while $$Mn^{3+}$$ is oxidising.

Analysis of Assertion:

• $$Cr^{2+}$$ has the electronic configuration $$[Ar]3d^4$$. It readily loses an electron to become $$Cr^{3+}$$ ($$[Ar]3d^3$$), which has a half-filled $$t_{2g}$$ sublevel — a very stable configuration. Hence, $$Cr^{2+}$$ acts as a strong reducing agent. $$\checkmark$$
• $$Mn^{3+}$$ has the configuration $$[Ar]3d^4$$. It readily gains an electron to become $$Mn^{2+}$$ ($$[Ar]3d^5$$), which has a half-filled d-shell — an extra stable configuration. Hence, $$Mn^{3+}$$ acts as a strong oxidising agent. $$\checkmark$$

Assertion A is TRUE.

Reason R: Extra stability to half-filled electronic configuration is observed compared to incompletely filled configuration.

Analysis of Reason:

This is a well-known fact. Half-filled configurations ($$d^3$$ in octahedral field and $$d^5$$) provide extra stability due to exchange energy. $$\checkmark$$

Reason R is TRUE.

Is R the correct explanation of A?

Yes. $$Cr^{2+}$$ ($$d^4$$) tends to become $$Cr^{3+}$$ ($$d^3$$, half-filled $$t_{2g}$$), making it reducing. $$Mn^{3+}$$ ($$d^4$$) tends to become $$Mn^{2+}$$ ($$d^5$$, half-filled d-shell), making it oxidising. The driving force in both cases is the extra stability of the half-filled configuration.

The correct answer is Option A: Both A and R are true and R is the correct explanation of A.