Which of the following statements are correct?
(A) The M³⁺/M²⁺ reduction potential for iron is greater than manganese.
(B) The higher oxidation states of first row d-block elements get stabilized by oxide ion.
(C) Aqueous solution of Cr²⁺ can liberate hydrogen from dilute acid.
(D) Magnetic moment of V²⁺ is observed between 4.4 - 5.2 BM.
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Test each statement against one fact: d-electron stability, oxide bonding, E°, and unpaired electrons.

1. Half-filled stability. Mn²⁺ is d⁵, a half-filled and very stable shell, so Mn³⁺ grabs an electron eagerly. For iron it is the other way round: Fe³⁺ is the d⁵ ion, so it is less keen to be reduced.

   Statement	Key fact	Verdict
   (A) Fe greater than Mn	E°(Mn³⁺/Mn²⁺) = +1.57 V, but E°(Fe³⁺/Fe²⁺) = +0.77 V	false: Mn is greater
   (B) Oxide stabilises high states	O²⁻ forms multiple bonds with the metal, as in Mn₂O₇ (Mn is +7) and CrO₄²⁻ (Cr is +6)	true
   (C) Cr²⁺ liberates H₂	E°(Cr³⁺/Cr²⁺) = -0.41 V, below 0, so Cr²⁺ reduces H⁺ to H₂	true
   (D) V²⁺ is 4.4 to 5.2 BM	V²⁺ is d³ with 3 unpaired electrons, so μ = √(3 × 5) = 3.87 BM	false

2. Magnetic moment check. The range 4.4 to 5.2 BM fits 4 unpaired electrons, where √24 = 4.90 BM. That is a d⁴ ion such as Cr²⁺, not the d³ ion V²⁺.
3. So. Only (B) and (C) are correct, which is option (a).

The idea to keep. For M³⁺/M²⁺ potentials, ask which ion is d⁵: a d⁵ product makes E° high. For magnetic moments, count unpaired electrons first and use √n(n+2).

Answer from the NTA final answer key. Solution by PrepWiser.