Always remember that the difference in ionization enthalpy of two successive states is approx or equal to $10$ to $\pu{15 eV}$ or less. The lower oxidation state is not stable; conversely if the difference is more than $\pu{15 eV}$, the lower oxidation state is more stable. Here the $3^\mathrm{rd}$ and $2\mathrm{nd}$ ionization enthalpy difference is more than $\pu{15 eV}$; that's why second ionization enthalpy is more stable
Ayush
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