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Q4.3:
Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?

Solution :

Theoretical Basis: Electronic Configuration and Ionization Enthalpy

The stability of the $+2$ oxidation state across the first half of the first-row transition elements (Scandium to Manganese) is dictated by the interplay between the sum of the first two ionization enthalpies ($\Delta_iH_1 + \Delta_iH_2$) and the hydration enthalpy of the resulting ions. [Per the Born-Haber Cycle principle].

Step 1: Analyzing Atomic Configurations ($3d^n 4s^2$)

The first-row transition elements possess a general configuration of $[Ar] 3d^{1-10} 4s^2$. Upon the formation of a $+2$ ion, the $4s$ electrons are removed first. The electronic configuration of the $M^{2+}$ ions is as follows:

  • $Sc^{2+}$ ($d^1$)
  • $Ti^{2+}$ ($d^2$)
  • $V^{2+}$ ($d^3$)
  • $Cr^{2+}$ ($d^4$)
  • $Mn^{2+}$ ($d^5$)

Step 2: Trends in Ionization Enthalpy

As the atomic number increases from $Sc$ ($Z=21$) to $Mn$ ($Z=25$), the effective nuclear charge ($Z_{eff}$) increases. This leads to a stronger attraction between the nucleus and the valence electrons. Consequently, the energy required to remove two electrons (sum of $\Delta_iH_1$ and $\Delta_iH_2$) increases. However, the stability of the $+2$ state is not determined by ionization energy alone, but by the relative energy cost versus the gain in stabilization energy.

Step 3: Stabilization via Exchange Energy and Symmetries

The $+2$ state becomes increasingly stable as we approach $Mn^{2+}$ due to the following factors:

  • Exchange Energy: $Mn^{2+}$ possesses a $3d^5$ configuration, which is a half-filled subshell. According to [Hund's Rule of Maximum Multiplicity], a half-filled $d$-orbital configuration confers extra stability due to the maximum number of exchange interactions.
  • Crystal Field Stabilization: In aqueous media, the stability is further governed by the hydration enthalpy of the metal ions. As the ionic radius decreases across the period due to increased nuclear charge, the hydration enthalpy becomes more exothermic, compensating for the increasing ionization energy.

Step 4: Comparative Data Analysis

Element Configuration ($M^{2+}$) Stability Trend
$Sc$ $d^1$ Low (tendency to reach $d^0$)
$Ti$ $d^2$ Increasing
$V$ $d^3$ Increasing ($t_{2g}^3$ symmetry)
$Cr$ $d^4$ High
$Mn$ $d^5$ Maximum ($d^5$ half-filled stability)
Atomic Number (Increasing) Stability of M²⁺ Sc Mn

Final Solution: The stability of the $+2$ oxidation state increases because the electronic configurations approach the stable half-filled $d^5$ shell ($Mn^{2+}$). The increasing sum of ionization enthalpies is effectively offset by the increased hydration enthalpies of the smaller ions and the exchange energy stabilization associated with the $d^5$ configuration.


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


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