Question

For the four successive transition elements $(Cr, Mn, Fe$ and $Co)$ , the stability of $+ 2$ oxidation state will be there in which of the following order? (At. no. $Cr = 24, Mn = 25, Fe = 26,Co = 27$ )

• A. Fe > Mn > Co > Cr
• B. Co > Mn > Fe > C
• C. Cr > Mn > Co > Fe
• D. Mn > Fe > Cr > Co

Answer 1

The Correct Option is D

To determine the order of stability of the +2 oxidation state for the four successive transition elements (Cr, Mn, Fe, \text{ and } Co), we need to analyze the electronic configurations and stability considerations of each element in this oxidation state.

1. Electronic Configuration:
   • Chromium (Cr, Z = 24): [Ar] 3d⁵ 4s¹
   • Manganese (Mn, Z = 25): [Ar] 3d⁵ 4s²
   • Iron (Fe, Z = 26): [Ar] 3d⁶ 4s²
   • Cobalt (Co, Z = 27): [Ar] 3d⁷ 4s²
2. +2 Oxidation State: In the +2 oxidation state, each element loses two electrons, primarily from the 4s orbital.
   • Cr²⁺: [Ar] 3d⁴
   • Mn²⁺: [Ar] 3d⁵
   • Fe²⁺: [Ar] 3d⁶
   • Co²⁺: [Ar] 3d⁷
   The stability in the +2 oxidation state is enhanced by either a half-filled (3d⁵) or completely filled (3d¹⁰) d-subshell, which offers additional stability due to symmetrical electron distribution and exchange energy.
3. Analysis of Stability:
   • The half-filled 3d⁵ subshell in Mn²⁺ provides extra stability due to its symmetrical configuration.
   • Fe²⁺ with a 3d⁶ configuration is also fairly stable due to its approach towards half-filling.
   • Cr²⁺ has a 3d⁴ configuration which is less stable compared to Mn²⁺ and Fe²⁺ but more stable than Co²⁺.
   • Co²⁺ with a 3d⁷ configuration is the least stable among the four, as it benefits neither from half-filling nor full stability.
4. Conclusion: The stability of the +2 oxidation state for these elements is in the order:
   Mn > Fe > Cr > Co
   Option 4, Mn > Fe > Cr > Co, is the correct answer.