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Q4.19:
Compare the stability of +2 oxidation state for the elements of the first transition series.

Solution :

Initial Electronic Configuration & Theoretical Foundation

The stability of the $+2$ oxidation state in the first transition series ($3d$-series: $Sc$ to $Zn$) is governed by the energy required to remove two electrons (sum of the first and second ionization enthalpies, $IE_1 + IE_2$) relative to the stabilization energy gained through hydration or lattice formation. The general electronic configuration for these elements is $(n-1)d^{1-10}ns^2$. The removal of two $ns$ electrons yields an $(n-1)d^n$ configuration.

Step 1: Trend in Ionization Enthalpies and Stability

Stability is inversely proportional to the total ionization energy ($IE_1 + IE_2$). As we move across the series, effective nuclear charge ($Z_{eff}$) increases, making the removal of electrons increasingly difficult. However, the exchange energy associated with half-filled ($d^5$) and fully-filled ($d^{10}$) configurations provides extra stability.

Ti Cr Mn Fe Zn Relative Stability of +2 State

Step 2: Analytical Breakdown by Element

Element $3d$ Configuration (+2 state) Stability Analysis
$Ti$ ($22$) $3d^2$ Stable, but loses stability toward $V$ and $Cr$.
$V$ ($23$) $3d^3$ Stable due to $t_{2g}$ subshell occupancy [Per Crystal Field Theory].
$Cr$ ($24$) $3d^4$ Less stable than $Mn^{2+}$; $Cr^{2+}$ is a strong reducing agent as it oxidizes to $Cr^{3+}$ ($3d^3$, $t_{2g}^3$ stability).
$Mn$ ($25$) $3d^5$ Highly Stable. The $d^5$ configuration is half-filled and highly symmetric, resulting in maximum exchange energy [Per Hund’s Rule and Quantum Mechanical Stability].
$Fe$ ($26$) $3d^6$ Stability decreases compared to $Mn^{2+}$; $Fe^{2+}$ easily oxidizes to $Fe^{3+}$ ($d^5$).
$Zn$ ($30$) $3d^{10}$ Highly Stable. The $d^{10}$ configuration represents a completely filled subshell, providing high inertness to further ionization.

Step 3: Comparative Conclusion

  • Maximum Stability: $Mn^{2+}$ ($d^5$) and $Zn^{2+}$ ($d^{10}$) are the most stable $+2$ ions due to half-filled and fully-filled $d$-orbital stability respectively.
  • Reducing Character: $Cr^{2+}$ and $Fe^{2+}$ are relatively less stable in the $+2$ state compared to $+3$ because the transition to $d^3$ or $d^5$ configurations releases significant exchange energy.
  • General Trend: Stability fluctuates based on the energy required to achieve stable electronic configurations; the $IE_1 + IE_2$ values generally increase across the series, but are offset by electronic configuration effects.

Final Solution: The stability of the +2 oxidation state follows the order: $Mn^{2+}$ ($d^5$) and $Zn^{2+}$ ($d^{10}$) exhibit the highest stability due to their half-filled and fully-filled $d$-subshells. Elements like $Cr^{2+}$ ($d^4$) and $Fe^{2+}$ ($d^6$) are less stable, acting as reducing agents because they tend to attain the more stable $d^3$ or $d^5$ configurations.


More Questions from Class 12 Chemistry The d- and f-Block Elements Exercises


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